Carbon nanotube biosensors

Abstract

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.

Keywords: biocompatibility; biosensing; cancer; carbon nanotube; fluorescence; functionalization; internalization.

Figure 1

Structure and models of carbon nanotubes in function of their number of walls. (A) Single-walled carbon nanotubes (SWNTs) structures in function of their chirality (zigzag, armchair, and chiral). (B) Model of double-walled carbon nanotubes (DWNTs). (C) Structure of multi-walled carbon nanotubes (MWNTs) made up of several concentric shells.

Figure 2

Applications of carbon nanotubes.

Figure 3

Exohedral and endohedral functionalization of CNTs. (A) Non-covalent surface functionalization of a buckytubes with a micellar surfactant or (B) with a shell-like surfactant wrapping around the nanotubes. In both cases, hydrophobic groups are evidenced by blue lines and hydrophilic ones by yellow stars. (C) Model of nanotubes filled with “peapods” (C₆₀). (D) DWNTs filled on all the length with molten FeI₂ (FeI₂@DWNTs).

Figure 4

Cellular internalization of carbon nanotubes via “nanoneedle” mechanism vs. endocytotic pathway, following in vivo injection. (A) Functionalized CNT rapidly penetrates the cell membrane directly to the nucleus, where it releases the cargo (red circles). (B) Functionalized CNT is internalized in the cell by endocytosis and delivered to the endosome, which matures to a lysosome. The accumulation into the lysosome causes swelling and rupture of the vesicle followed by the release of the functionalized CNT into the cytoplasm. The cargo is then able to diffuse through the cytoplasm.

Figure 5

Biosensor development timeline.

Figure 6

Electrochemical and Electronic CNT biosensors. (A) Typical design of an enzyme-based electrochemical biosensor. (B) SWNT electrically-contacted glucose oxidase electrode. (C) Schematic illustration of a label-free amperometric biosensor for PSA detection. (D) Schematic illustration of a microfluidic chip based on CNT electrodes (Yuichi et al., 2009).

Figure 7

Electrochemical and Electronic CNT biosensors for Cancer Detection. (A) Schematic illustration of the folic acid-targeted cytosensing strategy for an enhanced electrochemical detection of cancer cells using polydopamine-coated carbon nanotubes. (B) Schematic representation of an electrochemical DNA biosensor for cancer detection based on gold nanoparticles/aligned CNTs.

Figure 8

Immuno-CNT biosensors. (A) Schematic representation of an electrochemical CNT-immunosensor that combines a sensing interface for target detection with a sandwich-type electrochemical immunoassay for amplification of the signal. (B) Cytosensing immunosensor based on functionalization of SWNTs with RGDS peptides that recognize cell surface integrin receptors.

Figure 9

Optical CNT biosensors. (A) Schematic representation of a genetically-encoded FRET kinase biosensor: the donor (GFP) and acceptor (RFP) proteins are brought in close proximity which enables FRET following a phosphorylation-mediated intramolecular conformational change. (B) Schematic representation of non-genetic, environmentally-sensitive peptide-based biosensor: the probe environment is altered upon phosphorylation of the substrate. (C) CNT-biosensor based on wavelength shift of fluorescence upon target binding to a receptor conjugated into the SWNT. (D) Optical biosensor of ssDNA: fluorescence of a dye-labeled oligonucleotide is quenched upon non-covalent assembly with SWNT, until the ssDNA target binds and releases the labeled oligonucleotide from the SWNT. (E) Photoacoustic detection of integrins by indocyanine-labeled SWNT-biosensor conjugated with RGD peptides (Zerda et al., 2010).