Biofunctionalization of carbon nanofiber tips for scanning probe microscopy using thin Au coating and immobilized biorecognition molecules

Individual carbon nanofibers (CNFs) synthesized on the apices of tetrahedral-type Si cantilever tips via argon-ion (Ar⁺) bombardment, i.e., CNF-tipped scanning probe microscopy (SPM) probes, were sputter coated with a Au film without undergoing any obvious deformations. Indentations made using the Au-coated CNF tips showed that the buckling force of the tips increased with the thickness of the Au film. The CNF tip coated with a thin Au film less than approximately 8 nm in thickness elastically buckled when subjected to mechanical deflection, which was similar to the bare CNF tips. After annealing the Au-coated CNF-tipped SPM probes, we biofunctionalized them by immobilizing thiol-derivatized, single-stranded DNA (ssDNA) molecules, used as representative biorecognition molecules, on the Au(1 1 1)-coated surfaces. The ability of the immobilized ssDNA molecules to bind with complimentary RNA targets was confirmed via field-emission scanning electron microscopy and energy-dispersive X-ray spectroscopy-based analysis of quantum dots bound to the ssDNA/RNA hybrids formed on the surfaces of the biofunctionalized CNF-tipped SPM probes.     

Gold nanoparticles/water-soluble carbon nanotubes/aromatic diamine polymer composite films for highly sensitive detection of cellobiose dehydrogenase gene

An electrochemical sensor based on gold nanoparticles (GNPs)/multiwalled carbon nanotubes (MWCNTs)/poly (1,5-naphthalenediamine) films modified glassy carbon electrode (GCE) was fabricated. The effectiveness of the sensor was confirmed by sensitive detection of cellobiose dehydrogenase (CDH) gene which was extracted from Phanerochaete chrysosporium using polymerase chain reaction (PCR). The monomer of 1,5-naphthalenediamine was electropolymerized on the GCE surface with abundant free amino groups which enhanced the stability of MWCNTs modified electrode. Congo red (CR)-functionalized MWCNTs possess excellent conductivity as well as high solubility in water which enabled to form the uniform and stable network nanostructures easily and created a large number of binding sites for electrodeposition of GNPs. The continuous GNPs together with MWCNTs greatly increased the surface area, conductivity and electrocatalytic activity. This electrode structure significantly improved the sensitivity of sensor and enhanced the DNA immobilization and hybridization. The thiol modified capture probes were immobilized onto the composite films-modified GCE by a direct formation of thiol–Au bond and horseradish peroxidase–streptavidin (HRP–SA) conjugates were labeled to the biotinylated detection probes through biotin–streptavidin bond. Scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used to investigate the film assembly and DNA hybridization processes. The amperometric current response to HRP-catalyzed reaction was linearly related to the common logarithm of the target nucleic acid concentration in the range of 1.0 × 10⁻¹⁵–1.0 × 10⁻¹⁰ M, with the detection limit of 1.2 × 10⁻¹⁶ M. In addition, the electrochemical biosensor exhibited high sensitivity, selectivity, stability and reproducibility.     

Optimising DNA binding to carbon nanotubes by non-covalent methods

The use of carbon nanotubes as a gene delivery system has been extensively studied in recent years owing to its potential advantages over viral vectors. To achieve this goal, carbon nanotubes have to be functionalized to become compatible with aqueous media and to bind the genetic material. To establish the best conditions for plasmid DNA binding, we compare the dispersion properties of single-, double- and multi-walled carbon nanotubes (SWCNTs, DWCNTs and MWCNTs, respectively) functionalized with a variety of surfactants by non-covalent attachment. The DNA binding properties of the functionalized carbon nanotubes were studied and compared by electrophoresis. Furthermore, a bilayer functionalization method for DNA binding on SWCNTs was developed that utilized RNA-wrapping to solubilize the nanotubes and cationic polymers as a bridge between nanotubes and DNA.     

Facile preparation of size-controlled gold nanoparticles using versatile and end-functionalized thioether polymer ligands

At present, thiol ligands are generally used whenever the classical Brust-Schiffrin two-phase method is employed to prepare metal nanoparticles. In general, the previous research was mainly focused on utilizing small molecular thiol compounds or thiol polymers as the stabilizers in organic phase to obtain small sized and uniform gold nanoparticles (Au NPs). Such preparations are usually associated with the problems of ligand exchange on the nanoparticle's surface due to strong Au-thiol interaction. Herein, we report an approach to produce fairly uniform Au NPs with diameters about 2-6 nm using thioether end-functional polymer ligands (DDT-PVAc and PTMP-PVAc) as the capping agents. These nanoparticles are thoroughly characterized using DLS, TEM, UV-Vis spectroscopy and other complementary techniques. The results indicate that multidentate thioether polymeric ligands (PTMP-PVAc) lead to formation of smaller but special 'multimer' morphology in organic phase; whereas fairly uniform nanoparticles are produced using monodentate thioether functionalized ligands (DDT-PVAc). Further modification of such polymer ligands to introduce the hydrophilic functionalities realizes the phase transfer of Au NPs from organic to aqueous media.     

Fabrication of single-wall carbon nanotubes within the channels of a mesoporous material by catalyst-supported chemical vapor deposition

Diameter-controlled single-wall carbon nanotubes (SWCNTs) have been synthesized using Co, Fe/Co and Rh/Pd alloy nanoparticles trapped within the one-dimensional channels of a mesoporous materials (Folded Sheets Mesoporous material: FSM-16) by catalyst-supported chemical vapor deposition (CCVD) using ethanol as carbon source at 973-1173 K. The SWCNTs synthesized are characterized by transmission electron microscopy, Raman spectroscopy and photoluminescence spectroscopy. The yield, diameter distribution and quality of the SWCNTs strongly depend on the reaction temperature during CCVD. The product synthesized at 1173 K contains only SWCNTs, in marked contrast to those synthesized at lower temperatures. As the reaction temperature decreases, the relative abundance of multi-wall carbon nanotubes against SWCNTs significantly increases, whereas the mean diameter of SWCNTs increases as reaction temperature increases. The results show that a careful control of the reaction temperature is crucial to fabricate diameter-controlled SWCNTs from the channels of FSM-16.     

The growth of CdSe quantum dots on a single wall carbon nanotubes template without organic solvent and surfactant

A conventional synthesis of Cadmium selenide (CdSe) quantum dots (QDs) usually employs toxic organic solvents, and the synthesized CdSe QDs must be modified for dispersion in an aqueous solution. This modification often limits the application of CdSe QDs in biomedical fields. In this study, a simple method was developed to synthesize CdSe QDs on single wall carbon nanotubes (SWCNTs) employing the SWCNTs as a template to prevent the aggregation of the CdSe QDs in an aqueous solution without the addition of any organic reagent.Our newly developed synthetic procedure included the formation of SWCNTs with carboxyl groups (SWCNT-COOHs) followed by mixing these with the precursors of Cd and Se to obtain SWCNT-CdSe QDs. The resulting SWCNT-CdSe QDs were analyzed using spectrophotometry, transmission electron microscopy (TEM) and X-ray diffraction (XRD).Results showed that CdSe nanocrystals with a zinc blend structure could be synthesized on the SWCNT-COOHs. The average crystal size of the synthesized CdSe QDs was approximately 3 nm. The blue-shift of CdSe QDs powerfully emitted light at 550 nm as compared to the bulk CdSe at 730 nm. These CdSe QDs were synthesized in an aqueous environment without using toxic surfactants and are expected to have great potential as bio-labeling contrast agents in the future.     

A comparative study of electrochemical and biointerfacial properties of acid- and plasma-treated single-walled carbon-nanotube-film electrode systems for use in biosensors

The electrocatalytic and biointerfacial properties of acid- and O₂-plasma-treated single-walled carbon nanotube (SWCNT) electrodes were investigated. The SWCNT-modified electrodes were characterized using scanning electron microscopy and X-ray photoelectron spectroscopy. The electrochemical performance of these electrodes was analyzed by cyclic voltammetry and chronoamperometry. Glucose oxidase was covalently immobilized on the surface of the treated SWCNTs, and the analytical characteristics of the integrated glucose sensor were investigated using glucose as a target analyte. The plasma-activated SWCNT electrode exhibited a much higher sensitivity to the glucose and a lower detection limit than the acid-treated electrode, indicating that a larger amount of enzyme was immobilized on the plasma-treated SWCNT electrode than on the acid-treated electrode. This is due to the fact that the oxygenated functional groups are mainly located at the ends of the tubes in the acid-treated SWCNTs, while the plasma-treated SWCNTs have an even larger surface area available for enzyme immobilization owing to the functional groups covering the entire surface of the SWCNTs.     

Room-temperature synthesis of single-wall carbon nanotubes by an electrochemical process

Single-wall carbon nanotubes (SWCNTs) were produced by an electrochemical route by applying a small negative potential to a solution of acetic acid over a Au surface supporting Ni nanocatalysts. Ni nanocatalysts were grown electrochemically on Au surface and their particle sizes were controlled by deposition time. Raman spectroscopy and scanning probe microscopy observations of the catalyst and as-deposited samples and revealed that the catalyst structure strongly affects the SWCNT diameter distribution. The deposited carbon structure depended on the catalyst particle size and structure. Raman spectra confirmed the existence of selectively grown semiconducting SWCNTs with very narrow diameter distribution.     

Effect of substrate (ZnO) morphology on enzyme immobilization and its catalytic activity

In this study, zinc oxide (ZnO) nanocrystals with different morphologies were synthesized and used as substrates for enzyme immobilization. The effects of morphology of ZnO nanocrystals on enzyme immobilization and their catalytic activities were investigated. The ZnO nanocrystals were prepared through a hydrothermal procedure using tetramethylammonium hydroxide as a mineralizing agent. The control on the morphology of ZnO nanocrystals was achieved by varying the ratio of CH₃OH to H₂O, which were used as solvents in the hydrothermal reaction system. The surface of as-prepared ZnO nanoparticles was functionalized with amino groups using 3-aminopropyltriethoxysilane and tetraethyl orthosilicate, and the amino groups on the surface were identified and calculated by FT-IR and the Kaiser assay. Horseradish peroxidase was immobilized on as-modified ZnO nanostructures with glutaraldehyde as a crosslinker. The results showed that three-dimensional nanomultipod is more appropriate for the immobilization of enzyme used further in catalytic reaction.     

Controlled assembly of carbon nanotubes encapsulated with amphiphilic block copolymer

An amphiphilic diblock copolymer (PEtOz-PCL) based on hydrophilic poly(2-ethyl-2-oxazoline) (PEtOz) and hydrophobic poly(ε-caprolactone) (PCL) was adsorbed in aqueous phase on the surface of single-wall carbon nanotube to produce PEtOz-PCL-encapsulated SWCNTs (PEtOz-PCL/SWCNT) with the diameter about 30 nm. The Raman spectroscopy analysis indicated that the nanotubes were physically encapsulated by the block copolymer without chemical denaturation of the nanotube. PEtOz-PCL/SWCNTs exhibited pH-responsive reversible complexation with poly(acrylic acid) or poly(methacrylic acid) in aqueous phase due to the pH-dependent hydrogen bonding between the PEtOz outer shell of PEtOz-PCL/SWCNTs with carboxyl groups. In addition, by using PEtOz as a template for the formation of metal nanoparticles, Au and Pd nanoparticles were successfully hybridized with PEtOz-PCL/SWCNTs.     

Fabrication of free-standing graphene composite films as electrochemical biosensors

We report a simple fabrication method for large-scale free-standing graphene–gold nanoparticle and graphene-single wall carbon nanotube composite films by using a centrifugal vacuum evaporation followed by a thermal reduction process. The homogeneous mixture of a graphene oxide (GO) suspension with gold nanoparticle (Au NP) or single wall carbon nanotube (SWCNT) is self-assembled at the air/liquid interface, resulting in the multilayered GO–Au NP and GO–SWCNT composite films. The cross-sectional image reveals that the graphene layers are orderly stacked in the reduced GO–Au NP film, while the reduced GO–SWCNT film shows a randomly packed morphology due to the dominant π–π interaction between the side wall of SWCNTs and the GO surfaces. In particular, the reduced GO–Au NP film shows an increased electrode kinetics and cyclic voltammetric response in proportion to the amount of Au NPs, and 3-fold enhancement of anodic peak current was observed compared with that of the reduced GO films. We employed the reduced GO–Au NP film as a matrix to immobilize tyrosinase enzyme for phenol detection, and the phenol-induced electrochemical catalytic reaction can be monitored with 3-fold higher sensitivity than the reduced GO film, demonstrating great potential of graphene composite as an electrochemical enzyme biosensor for environmental pollutant screening.     

Understanding the preferential oxidation of carbon monoxide (PrOx) using size‐controlled Au nanocrystal catalyst

Removing CO from hydrogen streams is an important industrial process. The catalytic preferential oxidation of CO (PrOx) is a promising method for CO removal, leaving the hydrogen concentration unchanged. Here, the effect of size and support on the gold‐catalyzed PrOx reaction using size‐controlled Au nanocrystals (NCs) is investigated. For all supports, Au NC sizes of 2–5 nm show the highest rates, whereas for larger sizes rates drop. Ceria‐supported Au shows by far the best performance. By analyzing the dependency of the reaction rate on the NC diameter, the most active centers for CO oxidation on Au/CeO₂ are Au⁺ corner atoms at the interface with the support, resulting in 2.1 nm Au NCs supported on ceria reaching full O₂ conversion and CO selectivity of about 50%. Therefore, it is suggested that increasing the fraction of Au‐ceria interface sites would lead to the best performing materials for this reaction. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3159–3167, 2018     

Electrocatalytic oxidation of ascorbic acid using a single layer of gold nanoparticles immobilized on 1,6-hexanedithiol modified gold electrode

This paper describes the electrocatalytic oxidation of ascorbic acid (AA) in phosphate buffer solution by the immobilized citrate capped gold nanoparticles (AuNPs) on 1,6-hexanedithiol (HDT) modified Au electrode. X-ray photoelectron spectrum (XPS) of HDT suggests that it forms a monolayer on Au surface through one of the two SH groups and the other SH group is pointing away from the electrode surface. The free SH groups of HDT were used to covalently attach colloidal AuNPs. The covalent attachment of AuNPs on HDT monolayer was confirmed from the observed characteristic carboxylate ion stretching modes of citrate attached with AuNPs in the infra-red reflection absorption spectrum (IRRAS) in addition to a higher reductive desorption charges obtained for AuNPs immobilized on HDT modified Au (Au/HDT/AuNPs) electrode in 0.1 M KOH when compared to HDT modified Au (Au/HDT) electrode. The electron transfer reaction of [Fe(CN)₆]^4−/3− was markedly hindered at the HDT modified Au (Au/HDT) electrode while it was restored with a peak separation of 74 mV after the immobilization of AuNPs on Au/HDT (Au/HDT/AuNPs) electrode indicating a good electronic communication between the immobilized AuNPs and the underlying bulk Au electrode through a HDT monolayer. The Cottrell slope obtained from the potential-step chronoamperometric measurements for the reduction of ferricyanide at Au/HDT/AuNPs was higher than that of bare Au electrode indicating the increased effective surface area of AuNPs modified electrode. The Au/HDT/AuNPs electrode exhibits excellent electrocatalytic activity towards the oxidation of ascorbic acid (AA) by enhancing the oxidation peak current to more than two times with a 210 mV negative shift in the oxidation potential when compared to a bare Au electrode. The standard heterogeneous electron transfer rate constant (k_s) calculated for AA oxidation at Au/HDT/AuNPs electrode was 5.4 × 10⁻³ cm s⁻¹. The oxidation peak of AA at Au/HDT/AuNPs electrode was highly stable upon repeated potential cycling. Linear calibration plot was obtained for AA over the concentration range of 1–110 μM with a correlation coefficient of 0.9950. The detection limit of AA was found to be 1 μM. The common physiological interferents such as glucose, oxalate ions and urea do not show any interference within the detection limit of AA. The selectivity of the AuNPs modified electrode was illustrated by the determination of AA in the presence of uric acid.     

Dispersion of single-walled carbon nanotubes modified with poly-l-tyrosine in water

In this study, complexes composed of poly-l-tyrosine (pLT) and single-walled carbon nanotubes (SWCNTs) were produced and the dispersibility of the pLT/SWCNT complexes in water by measuring the ζ potential of the complexes and the turbidity of the solution were investigated. It is found that the absolute value of the ζ potential of the pLT/SWCNT complexes is as high as that of SWCNTs modified with double-stranded DNA (dsDNA) and that the complexes remain stably dispersed in the water at least for two weeks. Thermogravimetry analysis (TGA) and visualization of the surface structures of pLT/SWCNT complexes using an atomic force microscope (AFM) were also carried out.     

Fabrication of nanoelectrode ensembles by electrodepositon of Au nanoparticles on single-layer graphene oxide sheets

Nanoelectrode ensembles (NEEs) have been fabricated by the electrodeposition of Au nanoparticles (AuNPs) on single-layer graphene oxide (GO) sheets coated on a glassy carbon electrode (GCE). The fabricated NEEs show a typical sigmoidal shaped voltammetric profile, arising from the low coverage density of AuNPs on GCE and large distance among them, which can be easily controlled by varying the electrodeposition time. As a proof of concept, after the probe HS-DNA is immobilized on the NEEs through the Au-S bonding, the target DNA is detected with the methylene blue intercalator. Our results show that the target DNA can be detected as low as 100 fM, i.e. 0.5 amol DNA in 5 μL solution.     

Photodynamic effects of chlorin e6 attached to single wall carbon nanotubes through noncovalent interactions

Chlorin e6 (Ce6), a big heterocyclic aromatic molecule, is considered promising photosensitizer for photodynamic therapy (PDT). Here an efficient nano-photosensitizer delivery system based on noncovalent interactions between Ce6 and single wall carbon nanotubes (SWCNTs) is proposed. By utilization of high surface area of SWCNTs, Ce6 was loaded on them with a high drug loading content by noncovalent π–π interactions. Then, the Ce6–SWCNT complexes were wrapped by chitosan to improve aqueous solubility and biocompatibility. The chemical characteristics of Ce6–SWCNTs and chitosan–Ce6–SWCNTs were evaluated by different analysis methods, including transmission electron microscopy, UV–Visible absorption spectra, and Fourier transform infrared spectra. The high cellular uptake of chitosan–Ce6–SWCNTs was confirmed by flow cytometry and confocal laser scanning microscopy. According to the WST-1 assay, the chitosan–Ce6–SWCNTs exhibited low dark toxicity and efficient PDT efficacy to HeLa cancer cells. These results indicate that chitosan–Ce6–SWCNTs are a potential photosensitizer delivery system for PDT.     

Layer-by-layer assembly of single-walled carbon nanotube-poly(viologen) derivative multilayers and their electrochemical properties

Layer-by-layer (LBL) multilayers of oxidized single-walled carbon nanotubes (SWCNTs) and poly(octylviologen) derivative (POV) have been assembled on gold electrode surfaces. The assembling process was characterized by quartz crystal microbalance (QCM) and electrochemical measurements. The average mass change was about 0.726 and 0.381 μg for each assembly of SWCNTs and POV, respectively. Cyclic voltammograms of the LBL multilayer modified electrodes showed well-reversible redox waves centered at about −640 mV vs Ag/AgCl, corresponding to the normal redox reaction of viologens. These LBL multilayers were very stable in the air and 1 mol/l KCl electrolyte solution. The results of QCM, cyclic voltammograms and chronocoulomograms of the multilayer modified Au electrodes indicated that the oxidized SWCNTs could not only support the formation of stable multilayers but also act as an electron mediator between viologens and electrodes.     

Electrochemical detection of Cd ions by a self-assembled monolayer of 1,9-nonanedithiol on gold

The application of 1,9-nonanedithiol (NDT) self-assembled monolayer (SAM) on gold for the electrochemical determination of Cd²⁺ was studied. Interestingly, we found that a NDT SAM strongly affects the stripping wave of Cd, resulting in a sharp peak that was used for electroanalytical determination of Cd²⁺ in aqueous solutions. The different parameters, such as potential and time of deposition of Cd, were examined. Furthermore, polarization-modulated infrared reflection absorption spectroscopy (PM IRRAS) and X-ray photoelectron spectroscopy (XPS) were used for exploring the interaction between the deposited Cd and the thiol groups on Au. FTIR measurements clearly indicate that NDT is assembled in a disordered liquid type monolayer interacting with the Au electrode via both thiol moieties. XPS reveals that Cd is stripped at two different potentials and that the signal of sulfur is almost unchanged by deposition and desorption of Cd. All these finding allude to the interesting conclusion that Cd is deposited on Au lifting to some extent the thiol groups.     

Characterization of poly(vinylferrocenium) coated surfaces and their applications in DNA sensor technology

Poly(vinylferrocenium) (PVF⁺) modified gold (Au) electrode was developed in this study for the electrochemical sensing of deoxyribonucleic acid (DNA) hybridization based on the oxidation signals of polymer and guanine, and also for the electrochemical investigation of interaction of anticancer drug, mitomycin C (MC) and DNA immobilized onto PVF⁺ modified Au electrode. PVF⁺ modified Au electrode was prepared by electrooxidation of poly(vinylferrocene) PVF at +0.7 V versus Ag/AgCl reference electrode. The polymer modified electrode and DNA immobilized polymer modified electrode were characterized by X-ray photoelectron (XPS), Fourier transform infrared-attenuated total reflentance (FTIR-ATR) and alternating current (AC) impedance spectroscopy. For application studies, differential pulse voltammetry (DPV) technique was used.     

Effect of surface properties of silica nanoparticles on their cytotoxicity and cellular distribution in murine macrophages

In this study, complexes composed of poly-l-tyrosine (pLT) and single-walled carbon nanotubes (SWCNTs) were produced and the dispersibility of the pLT/SWCNT complexes in water by measuring the ζ potential of the complexes and the turbidity of the solution were investigated. It is found that the absolute value of the ζ potential of the pLT/SWCNT complexes is as high as that of SWCNTs modified with double-stranded DNA (dsDNA) and that the complexes remain stably dispersed in the water at least for two weeks. Thermogravimetry analysis (TGA) and visualization of the surface structures of pLT/SWCNT complexes using an atomic force microscope (AFM) were also carried out.