AFM imaging of functionalized double-walled carbon nanotubes

Analysis of Vibration Frequency of Carbon Nanotubes used as Nano-Force Sensors Considering Clamped Boundary Condition

Carbon nanotubes (CNTs) can be used as atomic force microscope (AFM) probes since they are ideal tip materials with a small diameter, high aspect ratio, and stiffness. In this study, a model of CNTs clamped in an elastic medium is proposed as nanoscale force sensing AFM probes. The relationship between vibration frequency and axial force of the CNT probe clamped in an elastic medium is analyzed based on the Euler-Bernoulli beam model and the Whitney-Riley model. The clamped length of CNTs, and the elastic modulus of elastic medium affect largely on the vibration and the buckling stability of a CNT AFM probe. The result showed that the sensitivity to vibration increases as the applied loads increase. The critical load in which the vibration frequency decreases rapidly, moving to large ones with decreasing ratio of length to diameter of CNTs. The theoretical investigation on the vibration frequency of CNT loaded in the axial direction would give a useful reference for designing a CNT used as a nano-force sensor.

A review on protein functionalized carbon nanotubes

Carbon nanotubes (CNTs) have been widely recognized and used for controlled drug delivery and in various other fields due to their unique properties and distinct advantages. Both single-walled carbon nanotubes (SWCNTs) and multiwalled (MWCNTs) carbon nanotubes are used and/or studied for potential applications in medical, energy, textile, composite, and other areas. Since CNTs are chemically inert and are insoluble in water or other organic solvents, they are functionalized or modified to carry payloads or interact with biological molecules. CNTs have been preferably functionalized with proteins because CNTs are predominantly used for medical applications such as delivery of drugs, DNA and genes, and also for biosensing. Extensive studies have been conducted to understand the interactions, cytotoxicity, and potential applications of protein functionalized CNTs but contradicting results have been published on the cytotoxicity of the functionalized CNTs. This paper provides a brief review of CNTs functionalized with proteins, methods used to functionalize the CNTs, and their potential applications.

Carbon nanotube biosensors

Nanomaterials possess unique features which make them particularly attractive for biosensing applications. In particular, carbon nanotubes (CNTs) can serve as scaffolds for immobilization of biomolecules at their surface, and combine several exceptional physical, chemical, electrical, and optical characteristics properties which make them one of the best suited materials for the transduction of signals associated with the recognition of analytes, metabolites, or disease biomarkers. Here we provide a comprehensive review on these carbon nanostructures, in which we describe their structural and physical properties, functionalization and cellular uptake, biocompatibility, and toxicity issues. We further review historical developments in the field of biosensors, and describe the different types of biosensors which have been developed over time, with specific focus on CNT-conjugates engineered for biosensing applications, and in particular detection of cancer biomarkers.

Applications of biosensing atomic force microscopy in monitoring drug and nanoparticle delivery

INTRODUCTION

The therapeutic effects of medicinal drugs not only depend on their properties, but also on effective transport to the target receptor. Here we highlight recent developments in this discipline and show applications of atomic force microscopy (AFM) that enable us to track the effects of drugs and the effectiveness of nanoparticle delivery at the single molecule level.

AREAS COVERED

Physiological AFM imaging enables visualization of topographical changes to cells as a result of drug exposure and allows observation of cellular responses that yield morphological changes. When we upgrade the regular measuring tip to a molecular biosensor, it enables investigation of functional changes at the molecular level via single molecule force spectroscopy.

EXPERT OPINION

Biosensing AFM techniques have generated powerful tools to monitor drug delivery in (living) cells. While technical developments in actual AFM methods have simplified measurements at relevant physiological conditions, understanding both the biological and technical background is still a crucial factor. However, due to its potential impact, we expect the number of application-based biosensing AFM techniques to further increase in the near future.

Nanotubes complexed with DNA and proteins for resistive-pulse sensing

We use a resistive-pulse technique to analyze molecular hybrids of single-wall carbon nanotubes (SWNTs) wrapped in either single-stranded DNA or protein. Electric fields confined in a glass capillary nanopore allow us to probe the physical size and surface properties of molecular hybrids at the single-molecule level. We find that the translocation duration of a macromolecular hybrid is determined by its hydrodynamic size and solution mobility. The event current reveals the effects of ion exclusion by the rod-shaped hybrids and possible effects due to temporary polarization of the SWNT core. Our results pave the way to direct sensing of small DNA or protein molecules in a large unmodified solid-state nanopore by using nanofilaments as carriers.

Buckling Instability of Carbon Nanotube Atomic Force Microscope Probe Clamped in an Elastic Medium

Carbon nanotubes (CNTs) can be used as atomic force microscope (AFM) probes due to their robust mechanical properties, high aspect ratio and small diameter. In this study, a model of CNTs clamped in an elastic medium is proposed as CNT AFM probes. The buckling instability of the CNT probe clamped in elastic medium is analyzed based on the nonlocal Euler–Bernoulli beam model and the Whitney–Riley model. The clamped length of CNTs, and the stiffness of elastic medium affect largely on the stability of CNT AFM probe, especially at high buckling mode. The result shows that the buckling stability of the CNT AFM probe can be largely enhanced by increasing the stiffness of elastic medium. Moreover, the nonlocal effects of buckling instability are investigated and found to be lager for high buckling mode. The theoretical investigation on the buckling stability would give a useful reference for designing CNT as AFM probes.

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.

Diameter-selective dispersion of double-walled carbon nanotubes by lysozyme

We have utilized lysozyme to non-covalently functionalize and disperse double-walled carbon nanotubes (DWNTs) in aqueous solution. Lysozyme preferentially binds and disperses DWNTs with larger diameters. This is a facile and effective method to fractionalize and enrich DWNTs with certain diameters.

Driving Forces and Consequences of the Adsorption of Proteins to Carbon Nanotubes

Different strategies used to biofunctionalize CNTs with proteins, from direct physical adsorption on pristine CNTs to chemical treatments to achieve covalent interaction, are described. The discussion is focused on the consequences of the adsorption process on the structure and properties of both proteins and CNTs. On this base, recent developments in CNTs-proteins based biosensors (electrochemical and optical) and drug delivery systems are reviewed.

AFM imaging of functionalized carbon nanotubes on biological membranes

Multifunctional carbon nanotubes are promising for biomedical applications as their nano-size, together with their physical stability, gives access into the cell and various cellular compartments including the nucleus. However, the direct and label-free detection of carbon nanotube uptake into cells is a challenging task. The atomic force microscope (AFM) is capable of resolving details of cellular surfaces at the nanometer scale and thus allows following of the docking of carbon nanotubes to biological membranes. Here we present topographical AFM images of non-covalently functionalized single walled (SWNT) and double walled carbon nanotubes (DWNT) immobilized on different biological membranes, such as plasma membranes and nuclear envelopes, as well as on a monolayer of avidin molecules. We were able to visualize DWNT on the nuclear membrane while at the same time resolving individual nuclear pore complexes. Furthermore, we succeeded in localizing individual SWNT at the border of incubated cells and in identifying bundles of DWNT on cell surfaces by AFM imaging.