Noncovalent sidewall functionalization of single-walled carbon nanotubes for protein immobilization

Molecular dynamics simulation of polyethylene on single wall carbon nanotube

Molecular dynamics simulations are carried out on the isothermal crystallization process of single polyethylene chains with different chain lengths on the single wall carbon nanotube. This process is summarized as two steps, i.e., adsorption and orientation, and the bond-orientational order parameter is used to show the details of this process. The results show that the attractive van der Waals interactions control the adsorption and orientation of polyethylene on single wall carbon nanotube, and as the chain length increases, more microstructures appear in the last ordered structure. The stems of the ordered structure align parallel to the single wall carbon nanotube axis.

Targeted nanoparticle-based drug delivery and diagnosis

Nanotechnology, or systems/device manufacture at sizes generally ranging between 1 and 100 nm, is a multidisciplinary scientific field undergoing explosive development. The genesis of nanotechnology can be traced to advances in medicine, communications, genomics and robotics. One of the greatest values of nanotechnology will be in the development of new and effective medical treatments (i.e. nanomedicine). This review focuses on the potential of nanomedicine as it relates to the development of nanoparticles for enabling and improving the targeted delivery of therapeutic and diagnostic agents. We highlight the use of nanoparticles for specific intra-compartmental analysis using the examples of delivery to malignant cancers, to the central nervous system, and across the gastrointestinal barriers.

Phase Transition and Optical Properties of DNA-Gold Nanoparticle Assemblies

We review recent work on DNA-linked gold nanoparticle assemblies. The synthesis, properties, and phase behavior of such DNA-gold nanoparticle assemblies are described. These nanoparticle assemblies have strong optical extinction in the ultraviolet and visible light regions; hence, the technique is used to study the kinetics and phase transitions of DNA-gold nanoparticle assemblies. The melting transition of DNA-gold nanoparticle assemblies shows unusual trends compared to those of free DNA. The phase transitions are influenced by many parameters, such as nanoparticle size, DNA sequence, DNA grafting density, DNA linker length, interparticle distance, base pairing defects, and disorders. The physics of the DNA-gold nanoparticle assemblies can be understood in terms of the phase behavior of complex fluids, with the colloidal gold interaction potential dominated by DNA hybridization energies.

Supramolecular chemistry on water-soluble carbon nanotubes for drug loading and delivery

We show that large surface areas exist for supramolecular chemistry on single-walled carbon nanotubes (SWNTs) prefunctionalized noncovalently or covalently by common surfactant or acid-oxidation routes. Water-soluble SWNTs with poly(ethylene glycol) (PEG) functionalization via these routes allow for surprisingly high degrees of pi-stacking of aromatic molecules, including a cancer drug (doxorubicin) with ultrahigh loading capacity, a widely used fluorescence molecule (fluorescein), and combinations of molecules. Binding of molecules to nanotubes and their release can be controlled by varying the pH. The strength of pi-stacking of aromatic molecules is dependent on nanotube diameter, leading to a method for controlling the release rate of molecules from SWNTs by using nanotube materials with suitable diameter. This work introduces the concept of "functionalization partitioning" of SWNTs, i.e., imparting multiple chemical species, such as PEG, drugs, and fluorescent tags, with different functionalities onto the surface of the same nanotube. Such chemical partitioning should open up new opportunities in chemical, biological, and medical applications of novel nanomaterials.

Liquid Crystal Behavior of Single-Walled Carbon Nanotubes Dispersed in Biological Hyaluronic Acid Solutions

We report the spontaneous liquid crystal phase separation of nanotubes (single-walled carbon nanotubes, SWNTs) stabilized in aqueous biological (hyaluronic acid, HA) solutions. Sonication of SWNTs in solutions of HA produced well-dispersed single-phase isotropic dispersions which, over time, phase separated into dispersions containing birefringent nematic domains in equilibrium with an isotropic phase. The time required for phase separation to occur was shown to depend on the concentration of SWNT and HA, with the attractive interactions between the SWNT and HA shifting the onset of the phase separation toward lower concentration. This phase separation is accompanied by an increase in the dispersion viscosity with this increase qualitatively matching the degree of phase separation. The formation of ordered phases in biological media can offer wide opportunities for processing conducting biomaterials with aligned and oriented domains.

Direct measurements of interactions between polypeptides and carbon nanotubes

The interactions of various polypeptides with individual carbon nanotubes (CNTs), both multiwall (MW) and single wall (SW), were investigated by atomic force microscopy (AFM). While adhesion forces arising from electrostatic attraction interactions between the protonated amine groups of polylysine and carboxylic groups on the acid-oxidized multi-wall carbon nanotubes (Ox-MWCNTs) dominate the interaction at a low pH, weaker adhesion forces via the hydrogen bonding between the neutral -NH2 groups of polylysine and -COO- groups of the Ox-MWCNTs were detected at a high pH. The adhesion force was further found to increase with the oxidation time for Ox-MWCNTs and to be negligible for oxidized single-wall carbon nanotubes (Ox-SWCNTs) because carboxylate groups were only attached onto the nanotube tips in the latter whereas onto both the nanotube tips and sidewall in the former. Furthermore, it was demonstrated that proteins containing aromatic moieties, such as polytryptophan, showed a stronger adhesion force with Ox-MWCNTs than that of polylysine because of the additional pi-pi stacking interaction between the polytryptophan chains and CNTs.

Low-potential nicotinamide adenine dinucleotide detection at a glassy carbon electrode modified with toluidine blue O functionalized multiwall carbon nanotubes

The toluidine blue O (TBO) functionalized multiwall carbon nanotubes (MWNTs) nanomaterials (TBO-MWNTs) were prepared by assembling TBO onto the surface of a MWNTs modified glassy carbon (GC) electrode. Also TBO-MWNTs modified GC electrodes exhibiting a strong and stable electrocatalytic response toward beta-nicotinamide adenine dinucleotide (NADH) were described. Compared with a bare GC electrode, the TBO-MWNTs modified GC electrodes could decrease the oxidization overpotential of NADH by 730 mV, with a peak current at 0.0 V, since there was a positively synergistic electrocatalytic effect between the MWNTs and TBO toward NADH. Furthermore, the TBO-MWNTs modified GC electrodes had perfect performances, such as a low detection limit (down to 0.5 microM), being very stable (the current diminutions is lower than 6% in a period over 35 min), a fast response (within 3 s), and a wide linear range (from 2.0 microM to 3.5 mM). Such an ability of TBO-MWNTs to promote the NADH electron-transfer reaction suggests great promise for dehydrogenase-based amperometric biosensors.

Direct measurements of the interaction between pyrene and graphite in aqueous media by single molecule force spectroscopy: understanding the pi-pi interactions

Pyrene derivatives can absorb onto the surface of carbon nanotubes and graphite particles through pi-pi interactions to functionalize these inorganic building blocks with organic surface moieties. Using single molecule force spectroscopy, we have demonstrated the first direct measurement of the interaction between pyrene and a graphite surface. In particular, we have connected a pyrene molecule onto an AFM tip via a flexible poly(ethylene glycol) (PEG) chain to ensure the formation of a molecular bridge. The pi-pi interaction between pyrene and graphite is thus indicated to be approximately 55 pN with no hysteresis between the desorption and adhesion forces.

Water-soluble full-length single-wall carbon nanotube polyelectrolytes: preparation and characterization

HiPco single-wall carbon nanotubes (SWNTs) have been noncovalently modified with ionic pyrene and naphthalene derivatives to prepare water-soluble SWNT polyelectrolytes (SWNT-PEs), which are analogous to polyanions and polycations. The modified nanotubes have been characterized with UV-vis-NIR, fluorescence, Raman and X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The nanotube-adsorbate interactions consist of pi-pi stacking interactions between the aromatic core of the adsorbate and the nanotube surface and specific contributions because of the substituents. The interaction between nanotubes and adsorbates also involves charge transfer from adsorbates to SWNTs, and with naphthalene sulfonates the role of a free amino group was important. The ionic surface charge density of the modified SWNTs is constant and probably controlled by electrostatic repulsion between like charges. The linear ionic charge density of the modified SWNTs is similar to that of common highly charged polyelectrolytes.

Nanocomposite of functionalized multiwall carbon nanotubes with nafion, nano platinum, and nano gold biosensing film for simultaneous determination of ascorbic acid, epinephrine, and uric acid

A unique bimetallic, nano platinum (Pt) with nano gold (Au) on nafion (NF) incorporated with functionalized multiwall carbon nanotubes (f-MWCNTs) composite film (f-MWCNTs-NF-PtAu) was developed by the potentiostatic method. The composite film exhibits promising efficient catalytic activity towards the oxidation of mixture of biochemical compounds and simultaneous measurement of ascorbate anion, epinephrine and urate anion in aqueous buffer solution (pH 6.75). Both, the cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used for the measurement of electroanalytical properties of neurotransmitters by means of composite film modified electrodes. Well-separated voltammetric peaks were obtained for ascorbate, epinephrine and urate anions with the peak separations of 0.222 and 0.131V. The composite film can also be produced on gold and transparent semiconductor indium tin oxide electrodes for different kinds of studies such as electrochemical quartz crystal microbalance (EQCM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The incorporation of Pt and Au onto the f-MWCNTs-NF was revealed by the EQCM technique and the morphology of the film was studied using SEM, AFM and scanning electrochemical microscopy (SECM) techniques. Further, extensive studies were carried out using SECM for obtaining the surface current topographic images of composite film modified electrodes, and these indicated the presence of f-MWCNTs-NF-PtAu composite film on the electrode.

Overoxidized polypyrrole film directed single-walled carbon nanotubes immobilization on glassy carbon electrode and its sensing applications

In this paper, the films of overoxidized polypyrrole (PPyox) directed single-walled carbon nanotubes (SWNTs) have been electrochemically coated onto glassy carbon electrode (GCE). Electroactive monomer pyrrole was added into the solution containing sodium dodecyl sulfate (SDS) and SWNTs. Then, electropolymerization was proceeded at the surface of GCE, and a novel kind of conducting polymer/carbon nanotubes (CNTs) composite film with the orientation of CNTs were obtained correspondingly. Finally, this obtained polypyrrole (PPy)/SWNTs film modified GCE was oxidized at a potential of +1.8 V. It can be found that this proposed PPyox/SWNTs composite film modified GCE exhibited excellent electrocatalytic properties for some species such as nitrite, ascorbic acid (AA), dopamine (DA) and uric acid (UA), and could be used as a new sensor for practical applications. Compared with previous CNTs modified electrodes, SWNTs were oriented towards the outside of modified layer by PPyox and SDS, which made the film easily conductive. Moreover, this proposed film modified electrode was more stable, selective and applicable.

One-step production of polymeric microtubes by co-electrospinning

Herein we demonstrate the ability to fabricate polymeric microtubes with an inner diameter of approximately 3 microm through co-electrospinning of core and shell polymeric solutions. The mechanism by which the core/shell structure is transformed into hollow fibers (microtubes) is primarily based on the evaporation of the core solution through the shell and is described here in detail. Additionally, we present the filling of these microtubes, thus demonstrating their possible use in microfluidics. We also report the incorporation of a protein (green fluorescent protein) within such fibers, which is of interest for sensorics.

Solution−Chemical Synthesis of Carbon Nanotube/ZnS Nanoparticle Core/Shell Heterostructures

A facile solution-chemical method has been developed to be capable of encapsulating a multiwalled carbon nanotube (MWCNT) with ZnS nanocrystals without using any bridging species. The thickness of the ZnS shell can be tuned easily by controlling the experimental conditions. The optical properties of the MWCNT/ZnS heterostructures were investigated using UV-vis absorption and photoluminescence spectroscopy. The optical absorption spectrum indicates that the band gap of ZnS nanocrystallites is 4.2 eV. On the basis of the photoluminescence spectrum, charge transfer is thought to proceed from ZnS nanocrystals to the nanotube in the ZnS-carbon nanotube system. These special heterostructures are very easily encapsulated within a uniform silica layer by a modified-Stöber process and still show better stability even after heat treatment at 400 degrees C, which makes them appealing for practical applications in biochemistry and biodiagnostics.

Solubilization of single-walled carbon nanotubes by using polycyclic aromatic ammonium amphiphiles in water--strategy for the design of high-performance solubilizers

We describe the design of polycyclic aromatic compounds with high performance that dissolve single-walled carbon nanotubes (SWNTs). Synthetic amphiphiles trimethyl-(2-oxo-2-phenylethyl)-ammonium bromide (1) and trimethyl-(2-naphthalen-2-yl-2-oxo-ethyl)-ammonium bromide (2) carrying a phenyl or a naphtyl moiety were not able to dissolve/disperse SWNTs in water. By contrast, trimethyl-(2-oxo-2-phenanthren-9-yl-ethyl)-ammonium bromide (3) solubilized SWNTs, although the solubilization ability was lower than that of trimethyl-(2-oxo-2-pyrene-1-yl-ethyl)-ammonium bromide (4) (solubilization behavior observed by using 4 was described briefly in reference 4a). Transmission electron microscopy (TEM), as well as visible/near-IR, fluorescence, and near-IR photoluminescence spectroscopies were employed to reveal the solubilization properties of 4 in water, and to compare these results with those obtained by using sodium dodecyl sulfate (SDS) and hexadecyltrimethylammonium bromide (HTAB) as solubilizers. Compound 4 solubilized both the as-produced SWNTs (raw-SWNTs) and purified SWNTs under mild experimental conditions, and the solubilization ability was better than that of SDS and HTAB. Near-IR photoluminescence measurements revealed that the chiral indices of the SWNTs dissolved in an aqueous solution of 4 were quite different from those obtained by using micelles of SDS and HTAB; for a SWNTs/4 solution, the intensity of the (7,6), (9,5), and (12,1) indices were strong and the chirality distribution was narrower than those of the micellar solutions. This indicates that the aqueous solution of 4 has a tendency to dissolve semiconducting SWNTs with diameters in the range of 0.89-1.0 nm, which are larger than those SWNTs (0.76-0.97 nm) dissolved in the aqueous micelles of SDS and HTAB.

A cell nanoinjector based on carbon nanotubes

Technologies for introducing molecules into living cells are vital for probing the physical properties and biochemical interactions that govern the cell's behavior. Here, we report the development of a nanoscale cell injection system (termed the nanoinjector) that uses carbon nanotubes to deliver cargo into cells. A single multiwalled carbon nanotube attached to an atomic force microscope (AFM) tip was functionalized with cargo via a disulfide-based linker. Penetration of cell membranes with this "nanoneedle" was controlled by the AFM. The following reductive cleavage of the disulfide bonds within the cell's interior resulted in the release of cargo inside the cells, after which the nanoneedle was retracted by AFM control. The capability of the nanoinjector was demonstrated by injection of protein-coated quantum dots into live human cells. Single-particle tracking was used to characterize the diffusion dynamics of injected quantum dots in the cytosol. This technique causes no discernible membrane or cell damage, and can deliver a discrete number of molecules to the cell's interior without the requirement of a carrier solvent.

Development of immunosensors using carbon nanotubes

With increasing reports on bioterrorism, avian flu, and other bio-threats, rapid and real time detection methods are highly warranted. Studies on developing highly sensitive immunosensors aiming at the early detection and clinical diagnoses of various diseases including cancer are undertaken all over the globe. Carbon nanotubes (CNTs) have been widely discussed as materials with enormous potential for a wide range of in vivo and in vitro bioapplications, ranging from drug delivery to highly sensitive biosensors, owing to their superior electronic and mechanical properties along with nanoscale dimensions. Though a lot of attention has been drawn toward carbon nanotubes for the past 15 years in academia and to a certain extent in industry, CNT-based immunosensors and other applications are still in the nascent stage, and there are many challenges to be overcome for the successful commercialization of the concepts. This article highlights on the recent developments and the possible impacts of carbon nanotube based immunosensors.

V-type nerve agent detection using a carbon nanotube-based amperometric enzyme electrode

An enzyme electrode for the detection of V-type nerve agents, VX (O-ethyl-S-2-diisopropylaminoethyl methylphosphonothioate) and R-VX (O-isobutyl-S-2-diethylaminoethyl methylphosphonothioate), is proposed. The principle of the new biosensor is based on the enzyme-catalyzed hydrolysis of the nerve agents and amperometric detection of the thiol-containing hydrolysis products at carbon nanotube-modified screen-printed electrodes. Demeton-S was used as a nerve agent mimic. 2-(Diethylamino)ethanethiol (DEAET) and 2-(dimethylamino)ethanethiol (DMAET), the thiol-containing hydrolysis product and hydrolysis product mimic of R-VX and VX, respectively, were monitored by exploiting the electrocatalytic activity of carbon nanotubes (CNT). As low as 2 microM DMAET and 0.8 microM DEAET were detected selectively at a low applied potential of 0.5 V vs Ag/AgCl at a CNT-modified mediator-free amperometric electrode. Further, the large surface area and the hydrophobicity of CNT was used to immobilize organophosphorus hydrolase mutant with improved catalytic activity for the hydrolysis of the P-S bond of phosphothiolester neurotoxins including VX and R-VX nerve gases to develop a novel, mediator-free, membrane-free biosensor for V-type nerve agents. The applicability of the biosensor was demonstrated for direct, rapid, and selective detection of V-type nerve agents' mimic demeton-S. The selectivity of the sensor against interferences and application to spiked lake water samples was demonstrated.

Aqueous dispersion, surface thiolation, and direct self-assembly of carbon nanotubes on gold

We report the efficient aqueous dispersion of pristine HiPco single-walled carbon nanotubes (SWNTs) with ionic liquid (IL)-based surfactants 1-dodecyl-3-methylimidazolium bromide (1) and 1-(12-mercaptododecyl)-3-methylimidazolium bromide (2), the thiolation of nanotube sidewalls with 2, and the controlled self-assembly of positively charged SWNT-1,2 composites on gold. Optical absorption spectra and resonance Raman (RR) data of obtained aqueous SWNT-1,2 dispersions are consistent with debundled and noncovalently functionalized nanotubes whose electronic properties have not been disturbed. Additionally, the dispersion of pristine nanotube material with surfactants 1 and 2 leads to a high degree of purification from carbonaceous particles. The chiralities of the 14 smallest semiconducting HiPco SWNTs in resonance with Raman excitation at 1064 nm (1.165 eV) were determined in SWNT-2 aqueous dispersion using UV-vis-NIR and RR spectra. X-ray photoelectron spectroscopy (XPS) and surface-enhanced resonance Raman scattering (SERRS) spectroscopy of SWNT-2 submonolayers on gold verified the encapsulation of individualized SWNTs with IL surfactants, the cleavage of S-S disulfide bonds formed in aqueous SWNT-2 suspensions, and the direct chemisorption of the SWNT-2 composite on bare gold via the Au-S bond. Aqueous dispersions of SWNTs with IL-based surfactants add biofunctionality to carbon nanotubes by imparting the positive surface charge necessary for interactions with cell membranes. Our technique, which purifies pristine nanotube material and produces water-soluble, positively charged nanotubes with pendent surface-active thiol groups, may also be translated to other carbon nanotubes and carbon nanostructures. Self-assembled, positively charged submonolayers of SWNTs can be further used for applications in cell biology and sensor technology.

Noncovalent functionalization of carbon nanotubes by fluorescein-polyethylene glycol: supramolecular conjugates with pH-dependent absorbance and fluorescence

We report the use of fluorescein-polyethylene glycol (Fluor-PEG) to non-covalently functionalize single-walled carbon nanotubes (SWNTs) for obtaining aqueous-soluble nanotube conjugates (Fluor-PEG/SWNT) and simultaneously affording fluorescence labels to nanotubes. We find serendipitously that fluorescein, a widely used fluorophore, can strongly adsorb onto the sidewall of the SWNTs likely via π-stacking, and the hydrophilic PEG chain imparts high aqueous solubility. Interaction between fluorescein and SWNT is pH dependent; it weakens as the pH is increased, causing the Fluor-PEG/SWNT conjugate to be less stable at high pHs. Fluorescein molecules bound to SWNTs exhibit interesting pH dependent optical absorbance and fluorescence properties that are distinct from free molecules, as a result of pH dependent interactions with SWNT sidewalls. Fluorescence emission from fluorescein adsorbed on SWNT is quenched by ~67%, but remains sufficient and useful as a fluorescent label. The utility of Fluor-PEG/SWNT as a simultaneous fluorescent marker and an intracellular transporter is demonstrated by uptake of Fluor-PEG/SWNT by mammalian cells and detection of fluorescence inside the cells. Raman detection of SWNTs in the cells is also carried out and used to prove the co-localization of fluorescein and SWNT.

Synthesis and solution behavior of carbon nanotubes decorated with amphiphilic block polyelectrolytes

The aqueous solubilization of carbon nanotubes (CNTs) with the aid of a block copolymer possessing one polyelectrolyte block (namely polystyrene-b-sodium (sulfamate/carboxylate polyisoprene)) is reported. The solubilization protocol, based on the co-dissolution of the copolymer and the CNTs, leads to the formation of supramolecular assemblies on the side walls of the tubes. Electron microscopy as well as infrared spectroscopy and thermogravimetric analysis were employed as meaningful probes to identify the adsorption of the polymer onto the surface of CNTs and the composition of the final hybrid material. Viscosity measurements on solutions of the copolymer decorated CNTs indicate that the polyelectrolyte effect, which is observed in the case of net polymers, is preserved in a lesser extent in the case of the copolymer/CNTs dispersions.

Optically modulated conduction in chromophore-functionalized single-wall carbon nanotubes

We demonstrate an optically active nanotube-hybrid material by functionalizing single-wall nanotubes with an azo-based chromophore. Upon UV illumination, the conjugated chromophore undergoes a cis-trans isomerization leading to a charge redistribution near the nanotube. This charge redistribution changes the local electrostatic environment, shifting the threshold voltage and increasing the conductivity of the nanotube transistor. For a approximately 1%-2% coverage, we measure a shift in the threshold voltage of up to 1.2 V. Further, the conductance change is reversible and repeatable over long periods of time, indicating that the chromophore-functionalized nanotubes are useful for integrated nanophotodetectors.

Hemoglobin niobate composite based biosensor for efficient determination of hydrogen peroxide in a broad pH range

Inorganic layered niobates (HCa2Nb3O10) were used as immobilization matrices of hemoglobin (Hb) because of their tunable interlayer spaces, large surface areas and good biocompatibilities. A pair of well-defined, quasi-reversible cycle voltammertric peaks were obtained at the Hb-HCa2Nb3O10 modified pyrolytic graphite electrode, suggesting that the layered niobates facilitate the electron transfer between the proteins and the electrode. Hb-HCa2Nb3O10 modified electrode exhibited electrocatalytic response for monitoring H2O2 with a large linear detection range from 25 microM to 3.0 mM and a relatively high sensitivity of 172 microA mM-1 cm-2. Based on the stabilizing effect of the layered niobates, Hb-HCa2Nb3O10 modified electrode can detect H2O2 in strongly acidic and basic solutions with pH of 1-12, which greatly expands the application fields of biosensors.

Label-free biosensor: a novel phage-modified Light Addressable Potentiometric Sensor system for cancer cell monitoring

Early diagnosis has become the most important factor influencing the cancer's curing efficiency in clinical medicine. Here we present a new way for detection of cancer markers and cancer cells based on phage-modified Light Addressable Potentiometric Sensor (phage-LAPS). Phages were immobilized on the Si(3)N(4) chip surfaces covalently. Using the back-illuminating system, this phage-LAPS is tested to detect human phosphatase of regenerating liver-3 (hPRL-3) in the concentrations of 0.04-400 nM, and the mammary adenocarcinoma cell (MDAMB231) in the concentrations of 0-105 mL. The maximum responding signal is about 10 and 60 microV, respectively. The results demonstrated that the system was more applicable to detection of cancer cell than that of cancer biomarkers. This work might show potential application in clinic assays of cancer and the study of phage-cell interaction.