Covalent Surface Modification Effects on Single‐Walled Carbon Nanotubes for Targeted Sensing and Optical Imaging

Mild covalent functionalization of single-walled carbon nanotubes highlighted by spectroscopic ellipsometry

Impact of vacancy defects in single-walled carbon nanotube on the structural properties of covalently attached aromatic diisocyanates

Prostate-specific antigen (PSA) is used to detect prostate cancer. RNA aptamers against this protein have been reported previously. In this work, the stability and interaction mechanism of aptamer with PSA and single-wall carbon nanotube (SWCNT) was studied by computational techniques. Molecular dynamics simulations were used to investigate the interaction of aptamer segments with the surfaces of SWCNT and prostate-specific antigen (PSA) protein in an aqueous solution by two steps. Firstly, the interaction of aptamer and SWCNT was studied by the molecular dynamic simulation method. Our results showed that an aptamer segment can attach to the surface of SWCNT with its helix axis perpendicularly. Our analysis shows that the π–π stacking interactions play an important role in the binding of the aptamer to carbon rings. Secondly, we analysed the interaction of PSA protein with aptamer in the aptamer–SWCNT assemble. We found that the 3D structure of aptamer in the presence of PSA protein and SWCNT was stable. Finally, this study provides evidence about the stability of aptamer as a sensor on the surface of SWCNT (as a fixed surface) for PSA protein as a ligand. This finding can help to improve new biosensors for detecting cancer protein markers.

Composite structures of single walled carbon nanotubes (SWNTs) and poly(3-octylthiophene) polymer were tested as field emitters. The structures studied were formed on n+-Si by mixing the two materials after they had been dissolved in chloroform. Three different structures were tested: (a) a SWNT mat, (b) a SWNT mat on a SWNT (30 wt %)–polymer composite and (c) a SWNT mat on pure polymer. Although the conductivity of the composite is very much lower than that of the nanotube mat, experiments show that the field emission threshold is lower and the emission current higher from structure (b) compared to the pure SWNT mat on n+-Si. A comparison between the field enhancement factors and the expected aspect ratios shows that enhancement due to an additional field enhancement mechanism other than that due to the geometrical shape of the SWNTs must exist. We propose that the field due to the surface potential modification at the triple junction between SWNTs, polymer and vacuum is adequate to create additional field emission from such sites. This mechanism can also be used to explain previously reported field emission characteristics of CNx nanocomposite films.

Methylene Blue (MB) is a water-soluble dye which can physically absorb on single-wall carbon nanotube (SWCNT) surface. We discovered the dye can quench SWCNT fluorescence when it is added to a SWCNT dispersions in water. At the same time its own fluorescence is quenched. Analyzing the parameters of both these quenching processes we observed that the attachment of the dye to a SWCNT depends strongly on the quality of the SWCNT coating. As a result the quenching of specific SWCNT species can be used as a specific indicator of the species “protection” and its affinity to a dispersing agent. We have observed that the SWCNT quenching by MB is relatively weak in a case of good ionic surfactants, and such a quenching process does not indicate any specific or unexpected trends. Lowing stability of the protective layer due to lower surfactant concentration or unfavorable pH was causing an increasing quenching. More specific effects were detected with more selective process or SWCNT suspended with single-stranded DNA (ssDNA). Relative quenching of SWCNT species in this case was not constant, and could be used as a measure of adhesion strength or density of the SS-DNA on these species. The process of MB attachment was found to be dynamic, and it be described by an temporal attachment of MB molecules to a SWCNT. A quenching model was developed, resulting in a mathematical description with only two parameters. One of them is the adhesion strength (or binding energy), and another is the available volume (space) near a nanotube surface. The most obvious parameter, the available volume, was increasing in a case of a weaker (sparse or thin) coating of SWCNT surface by ssDNA, that was associated with a lower affinity of such ssDNA type to a SWCVNT species. The volume was becoming much smaller with a good ssDNA-SWCNT affinity, preventing in this way the interaction of the quencher with this SWCNT species. The studied process has appeared to be a simple and reliable tool for quick comparison of the affinity between specific SWCNT and ssDNA in bulk SWCNT solutions. Physical and mathematical models can also help to understand and analyze a wider variety of interactions between SWCNT and suspending agents or wrappers. The attachment of external ionic molecules, such as MB, may also help in development of SWCNT separation methods.

【Recent studies have been reported the presence of Endocrine Disrupting Compounds, Pharmaceuticals and Personal Care Products (EDC/PPCPs) in surface and wastewater, which could potentially affect to the complicate behavior in coupled presence of nano-colloid particles and surfactants (adsorption, dispersion, and partitioning). In this study, the adsorption of EDC/PPCPs by Single Walled Carbon Nanotubes (SWNTs) as a representative of nano-particles in cationic surfactant solutions were investigated. Hydrophobic interactions ( ${\pi}-{\pi}$ Electron Donor-Acceptor) have been reported as a potential adsorption mechanisms for EDC/PPCPs onto SWNTs. Generally, the adsorptive capacity of the relatively hydrophobic EDC/PPCPs onto SWNTs decreased in the presence of cationic surfactant (Cetyltrimethyl Ammonium Bromide, CTAB). This study revealed that the competitive adsorption occurred between CTAB cations and EDC/PPCPs by occupying the available SWNT surface (CTAB adsorption onto SWNTs shows five-regime and maximum adsorption capacity of 370.4 mg/g by applying the BET isotherm). The adsorption capacity of $17{\alpha}$ -ethinyl estradiol (EE2) on SWNT showed the decrease of 48% in the presence of CTAB. However, the adsorbed naproxen (NAP) surely increased by forming hemimicelles and resulted in a favorable media formation for NAP partition to increase SWNTs adsorption capacity. The adsorbed NAP increased from 24 to 82.9 mg/g after the interaction of CTAB with NAP. The competitive adsorption for EDC/PPCPs onto SWNTs is likely to be a key factor in the presence of cationic surfactant, however, NAP adsorption showed a slight competition through $CH_3-CH_3$ interaction by forming hemimicelles on SWNT surface.】

The interaction of molecules and material surfaces is one of the fundamental topics in surface science. Also, understanding the behavior of molecules in nano-space is increasingly important in nano- and biotechnologies. A single-walled carbon nanotube (SWNT) is a monolayered material composed of all surface atoms, which provide a simple van der Waals potential originated from a single layer both for the outer surface (quasi 2D) and inner nano-space (quasi 1D). Furthermore, the quasi 1D electronic structure of SWNT allows resonant optical transitions, which are sensitively affected by the surrounding environment of SWNT. SWNTs are thus an ideal material for examining the behaviors of molecules on the surface or in the nano-space by means of optical spectroscopy. We use a singly suspended SWNT between mesa structures to minimize the interactions with the substrate and other nanotubes, and expose it directly to the ambient gas. Semiconducting SWNTs suspended in space exhibit resonant photoluminescence (PL) and Raman scattering, and their optical responses depend on the state of molecules adsorbed on the SWNT surface or encapsulated in the tube [1-3]. Therefore, gas molecule adsorption/desorption on the SWNT surface or inside of the tube can be probed by PL and Raman spectroscopy. We have found that water molecules form an adsorption layer of two-monolayer thickness on the hydrophobic SWNT surface as a result of hydrogen-bonding network of water molecules [2]. The water adsorption layer has a sizable effect on the radial breathing mode frequency in Raman scattering [4]. The phase of water molecules can also affect those properties of SWNTs. The phase diagrams of the water both outside and inside of SWNT will be constructed based on the results of spectroscopy. Furthermore, PL peak energies from hybrids of single-stranded DNA and SWNT were found to depend on the DNA base type, namely thymine, adenine, cytosine, and guanine [5]. The base type dependence was attributed to the adsorption energy of the DNA bases on the SWNT surface. [1] S. Chiashi, et al. Nano Lett. 8, 3097 (2008). [2] Y. Homma, et al. Phys. Rev. Lett. 110, 157402 (2013). [3] S. Chiashi, et al. J. Phys. Chem. Lett. 5, 408 (2014). [4] S. Chiashi, et al. Phys. Rev. B 91, 155415 (2015). [5] M. Ito, et al. J. Phys. Chem. C 119, 21141 (2015).

Electrochemical durability investigation of single-walled and multi-walled carbon nanotubes under potentiostatic conditions

The mechanism of CO oxidation by O2 on a single Au atom supported on pristine, mono atom vacancy (m), di atom vacancy (di) and the Stone Wales defect (SW) on single walled carbon nanotube (SWCNT) surface is systematically investigated theoretically using density functional theory. We determine that single Au atoms can be trapped effectively by the defects on SWCNTs. The defects on SWCNTs can enhance both the binding strength and catalytic activity of the supported single Au atom. Fundamental aspects such as adsorption energy and charge transfer are elucidated to analyze the adsorption properties of CO and O2 and co-adsorption of CO and O2 molecules. It is found that CO binds stronger than O2 on Au supported SWCNT. We clearly demonstrate that the defected SWCNT surface promotes electron transfer from the supported single Au atom to O2 molecules. On the other hand, this effect is weaker for pristine SWCNTs. It is observed that the high density of spin-polarized states are localized in the region of the Fermi level due to the strong interactions between Au (5d orbital) and the adjacent carbon (2p orbital) atoms, which influence the catalytic performance. In addition, we elucidate both the Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms of CO oxidation by O2. For the LH pathway, the barriers of the rate-limiting step are calculated to be 0.02 eV and 0.05 eV for Au/m-SWCNT and Au/di-SWCNT, respectively. To regenerate the active sites, an ER-like reaction occurs to form a second CO2 molecule. The ER pathway is observed on Au/m-SWCNT, Au/SW-SWCNT and Au/SWCNT in which the Au/m-SWCNT has a smaller barrier. The comparison with a previous study (Lu et al., J. Phys. Chem. C, 2009, 113, 20156-20160.) indicates that the curvature effect of SWCNTs is important for the catalytic property of the supported single Au. Overall, Au/m-SWCNT is identified as the most active catalyst for CO oxidation compared to pristine SWCNT, SW-SWCNT and di-SWCNT. Our findings give a clear description on the relationship between the defects in the support and the catalytic properties of Au and open a new avenue to develop carbon nanomaterial-based single atom catalysts for application in environmental and energy related fields.

The assembly of sodium polystyrene sulfonate (Na+-PSS) at the surface of single-walled carbon nanotubes (SWNTs) in pH 3 aqueous solution is described. Rather than forming linear or sheet-like chain morphologies over SWNT surfaces, Na+-PSS adopts a spherically collapsed conformation believed to be the result of cation (either Na+ or H+) condensation onto the ionized polymer chain. It is well reported that cations (and also anions) adsorb preferentially onto single-walled and multi-walled carbon nanotube surfaces leading to an increased ion concentration in the near surface regions relative to the bulk solution. This work provides experimental evidence for preferentially absorbed cation condensation onto PSS anions until those cations are spaced at distances corresponding to the Bjerrum length ( B), as defined by the Manning theory of ion condensation, at the SWNT surface. The resulting electrostearic repulsions allow the SWNTs to remain suspended for days. Furthermore , coulombic repulsion among SWNT bundles after cation adsorption alone is not sufficient to form stable suspensions—but rather the stearic repulsions associated with spherically collapsed PSS at the nanotube surface is responsible for suspension stability. It is believed that the ultrasonic agitation drives cations into the small spaces between SWNT bundles and coulombic potential attracts the PSS to those regions.

Single-walled carbon nanotubes (SWCNTs) are near-infrared fluorescent substrates used in biological sensing, imaging, and delivery applications. These constructs are composed of SWCNTs in concert with an associated corona phase surrounding the nanotube. In this manner, noncovalent adsorption of polymers onto SWCNTs is typically implemented to disperse hydrophobic SWCNTs in solution, while also providing a functional handle such as a molecular recognition element that confers analyte specificity, giving rise to a nanosensor. For example, single-stranded DNA (ssDNA)-functionalized SWCNTs have been developed in which distinct ssDNA sequences confer specificity to different neurotransmitters for real-time brain sensing. However, this initial corona phase may be replaced by other biomolecules, predominantly proteins, once the SWCNT-based constructs are applied in the actual biological system of interest. Consequently, function of these and other nanoparticle-based biosensors is often hindered by protein corona formation, in which adsorbed proteins from the surrounding biofluid preclude interaction of the sensor with the target analyte. Establishing rational design rules to inform development and refinement of such nanosensors requires an understanding of both the initial nanoscale architectures and compositional changes upon application in complex biofluids.Herein, we describe methods and outcomes to investigate the carbon nanotube corona phase, both initially and upon interaction with proteins in the surrounding solution. To study the as-designed corona, we demonstrate a direct mode of measuring in-solution nucleic acid geometries on SWCNTs via X-ray scattering interferometry (XSI). This technique leverages the interference pattern produced by ordered gold nanoparticle tags conjugated to the DNA on the SWCNT surface. Using XSI, we quantify surface-adsorbed nucleic acid morphologies, ionic strength-dependent conformational changes, and the dopamine sensing mechanism of a DNA-SWCNT nanosensor, supported by ab initio modeling. To study the protein corona-covered SWCNT, we expand upon our previous work profiling protein adsorption on mixed-chirality SWCNTs that revealed the composition and driving forces of protein corona formation. We now characterize the protein corona across a panel of different nanosensor-relevant ssDNA sequences and single-chirality SWCNT substrates. Taken together, we aim to elucidate both the initial and end-state SWCNT surface to guide future nanobiotechnology design strategies.

We report molecular dynamics simulations of DNA adsorption on a single-walled carbon nanotube (SWNT) in an aqueous environment. We have modeled a DNA segment with 12 base pairs (Dickerson dodecamer) and a (8,8) SWNT in water, with counterions to maintain total charge neutrality. Simulations show that DNA binds to the external surface of an uncharged or positively charged SWNT on a time scale of a few hundred picoseconds. The hydrophobic end groups of DNA are attracted to the hydrophobic SWNT surface of uncharged SWNTs, while the hydrophilic backbone of DNA does not bind to the uncharged SWNT. The binding mode of DNA to charged SWNTs is qualitatively different from uncharged SWNTs. The phosphodiester groups of the DNA backbone are attracted to a positively charged SWNT surface while DNA does not adsorb on negatively charged SWNTs. There is no evidence for canonical double-stranded DNA wrapping around either charged or uncharged SWNTs on the very short time scales of the simulations. The adsorption process appears to have negligible effect on the internal stacking structure of the DNA molecule but significantly affects the A to B form conversion of A-DNA. The adsorption of A-DNA onto an uncharged SWNT inhibits the complete relaxation of A-DNA to B-DNA within the time scale of the simulations. In contrast, binding of the A-DNA onto a positively charged SWNT may promote slightly the A to B conversion.

Single-walled carbon nanotubes (SWCNTs) exhibit nonphotobleaching, near-infrared (NIR) fluorescence suitable for bioimaging applications. In particular, SWCNT fluorescence quenching induced by biopolymer dispersants facilitates flexible design of molecular nanosensors, yet challenges remain in analyte selectivity and lack of rational design strategies. A sought-after alternative to haphazard molecular modulation of SWCNT-based fluorescence is to couple the movement of a quencher to the SWCNT surface, enabling fluorescence energy transfer to modulate molecular recognition with high selectivity. This study presents the rational design of SWCNT-based nanosensors with fluorescence energy transfer, leveraging the unique properties of methylene blue (MB) proximity-mediated fluorescence quenching. MB-SWCNT-based nanosensors exhibit 1- stability in redox environments and 2- analyte-specific displacement-driven fluorescence modulation. By designing hybridization-induced displacement of MB-conjugated ssDNA from the SWCNT surface, we calculate that SWCNT fluorescence modulation can occur within a 6.8 nm fluorescence resonance energy transfer distance from the SWCNT surface and develop a robust and versatile platform to synthesize oligonucleotide nanosensors with tunable ΔF/F0 of up to 150%. Building upon this strategy, we developed four distinct nanosensors capable of selectively detecting tobacco mosaic virus (TMV) viral RNA fragments, which successfully differentiated TMV-infected plants from mock controls. Finally, we demonstrate the potential expansion of our design to target a wider scope of biomolecules using the biotin-avidin system as a model. Taken together, our study presents a generalizable platform that enables rational engineering of SWCNT NIR fluorescence intensity through MB distance-dependent fluorescence energy transfer, overcoming the intrinsic selectivity challenges of current SWCNT nanosensors.

The capability of anti‐Stokes/Stokes Raman spectroscopy to evaluate chemical interactions at the interface of a conducting polymer/carbon nanotubes is demonstrated. Electrochemical polymerisation of the monomer 3,4‐ethylenedioxythiophene (EDOT) on a Au support covered with a single‐walled carbon nanotube (SWNT) film immersed in a LiClO4/CH3CN solution was carried out. At the resonant optical excitation, which occurs when the energy of the exciting light coincides with the energy of an electronic transition, poly(3,4‐ethylenedioxythiophene) (PEDOT) deposited electrochemically as a thin film of nanometric thickness on a rough Au support presents an abnormally intense anti‐Stokes Raman spectrum. The additional increase in Raman intensity in the anti‐Stokes branch observed when PEDOT is deposited on SWNTs is interpreted as resulting from the excitation of plasmons in the metallic nanotubes. A covalent functionalisation of SWNTs with PEDOT both in un‐doped and doped states takes place when the electropolymerisation of EDOT, with stopping at +1.6 V versus Ag/Ag+, is performed on a SWNT film deposited on a Au plate. The presence of PEDOT covalently functionalised SWNTs is rationalised by (1) a downshift by a few wavenumbers of the polymer Raman line associated with the symmetric CC stretching mode and (2) an upshift of the radial breathing modes of SWNTs, both variations revealing an interaction between SWNTs and the conjugated polymer. Raman studies performed at different excitation wavelengths indicate that the resonant optical excitation is the key condition to observe the abnormal anti‐Stokes Raman effect. Copyright © 2010 John Wiley & Sons, Ltd.

The synthesis of covalent multi-functionalized single-walled carbon nanotubes (SWNT) via the combination of two diazonium salt addition reactions with tetrabutylammonium hydroxide to form carbon nanotube salts was carried out. The formation of carbon nanotube salts allows SWNT to be easily dissolved in common solvents, which increases the processing capability of the SWNT. The dissolutions of some of the obtained products can be increased three-fold through multi-functionalization. Thus tetrabutylammonium cation plays an extraordinarily important role; i.e., balancing the charges on the functionalized SWNT surface and de-bundling of the functionalized SWNT. This approach provides a useful way of preparing some SWNT-containing biomaterials. The multi-functionalized SWNT were confirmed by standard techniques used for carbon nanotube characterization.