**4. Carbon nanotubes for biomedical imaging and detection**

The well-ordered molecular structure attributes CNTs with multiple distinct optical proper‐ ties, include strong NIR absorption, photoluminescence and Raman shift [92]. Structurally, SWNTs can be viewed as a cylinder rolled up by one layer of graphene sheet. TThere are infinite numbers of ways to roll a graphene sheet into a cylinder. Depending on different ways of wrapping, the particular nanotube could be metallic or semi-conductive. Individual semi-conductive SWNTs with appropriate chirality can generate a small band gap fluores‐ cence of 1 eV, which corresponds to NIR range (900-1600 nm), where biological tissues have very low absorption, scattering, and autofluorescence, and therefore, are very useful for bio‐ logical imaging. In the other side, the inherent graphene structure provides SWNTs with specific Raman scattering signature [93], which is strong enough for use in-vivo imaging. All these optical properties offer opportunities for SWNTs as contrast agents for near-infrared (NIR) photoluminescence imaging [94, 95], Raman imaging and optical absorption agent for photoacoustic imaging [96-98].

been investigated in murine tumor model [85]. Raman spectroscopy image of excised tissues confirmed efficient targeting of αv*β*3 integrin positive U87MG tumor by RGD [85]. This study also disclosed that CNTs have relatively long circulation time, and rapid renal clear‐ ance, which makes SWNTs an attractive diagnostic and therapeutic delivery vehicle. Zevale‐ ta et al further developed a Raman microscope capable of noninvasive in-vivo evaluation of tumors in mice with RGD-labeled SWNTs. Using the dynamic Raman microscope, pharma‐ cokinetics of SWNTs in the tumor was evaluated immediately following an intravenous in‐ jection of SWNTs. Raman spectral analysis revealed effectiveness of the RGD nanotubes to the integrin expressing U87MG tumor. The noninvasive Raman imaging results were com‐

Carbon Nanotubes for Use in Medicine: Potentials and Limitations

http://dx.doi.org/10.5772/51785

299

Photoacoustic imaging is an optical imaging technique that combines high optical absorp‐ tion contrast with diffraction-limited resolution of ultrasonic imaging, which allows deeper tissues to be viewed in living subjects. In photoacoustic imaging, short pulses of stimulating radiation are absorbed by tissues, resulting a subsequent thermal expansion and ultrasonic emission that can be detected by highly sensitive piezoelectric devices. However, many dis‐ eases, for example cancer, in their early stages, do not exhibit a natural photoacoustic con‐ trast, therefore administering an external photoacoustic contrast agent is necessary. Owing to the strong light absorption characteristic [77], the CNTs can be utilized as photoacoustic contrast agents. De la Zerdaet. al, in the first time, applied SWNTs for in-vivo imaging of tumors in mice [103]. In this study, SWNTs was surface modified by PL-PEG and further conjugated with cyclic RGD peptides for targeting αvβ3 integrin on cancer cells. Intrave‐ nous injection of these cyclic RGD functionalized CNTs to mice bearing tumors showed eight times stronger photoacoustic signal in the tumors than mice injected with non-targeted CNTs. This study suggested that photoacoustic imaging using targeted SWNTs could con‐ tribute to non-invasive in-vivo cancer imaging [103]. Similarly, in another study, SWNTs functionalized with antibody against αvβ3 integrin for photoacoustic imaging of human

**5. Selected examples for preparation of carbon nanotube based**

In the above CNTs for drug delivery section, we described the functionalization of CNTs with drugs or targeting molecules. These preparations are usually applied directly via intra‐ venous delivery route, which is the most widely used route of drug administration. Alterna‐ tive to the intravenous drug administration, some other routes of drug administrations are also important for certain specific applications. Different formulations of CNT-based thera‐ peutics have also been developed to suit the specific routes of administration. Here, we re‐ port several novel cases of CNTs applications for oral and transdermal delivery routes.

pared with excised tissues and shows consistency [97].

**4.3. Photoacaustic imaging**

glioblastoma tumors in nude mice [104].

**therapeutics**

#### **4.1. Photoluminescence imaging**

NIR photoluminescence of micelle encapsulated SWNTs was firstly discovered by O.Connel et al [7]. The single-particle dispersion of individual nanotubes was prepared by ultrasoni‐ cally agitating of raw SWNTs in SDS. The tube bundles, ropes, and residual catalyst were removed by ultracentrifugation, since the aggregation of nanotubes would quench fluores‐ cence. One advantage of the photoluminescence of SWNTs over organic fluorescence dyes is that SWNTs have no apparent photobleaching, and therefore, the SWNTs could be a power‐ ful tool for tracking changes in living system. Researchers have applied NIR photolumines‐ cence of SWNTs for tracking endocytosis and exocytosis of SWNTs in NIH-3T3 cells in real time [94, 95]. Moreover, conjugation of antibodies to SWNTs surface allowed specific cell targeting. They have shown that, with conjugation of anti-CD20, SWNTs selectively recog‐ nized CD20 cell surface receptor on B-cells with little binding to receptor negative T-cells. Similarly, with conjugation of Herceptin, SWNTs only recognize HER2/neu positive breast cancer cells. The selective binding of SWNTs was detected by intrinsic NIR photolumines‐ cence of nanotubes. This technique allows deep tissue penetration and high-resolution intra‐ vital microscopy imaging of tumor vessels beneath thick skin [99, 100].

#### **4.2. Raman shift imaging**

Raman spectroscopy is a sensitive analytical tool for biological samples. It also has advan‐ tages of resistance to autofluorescence and photobleaching, high spatial resolution, and small sample size [101]. CNTs exhibit strong resonance Raman scattering with several dis‐ tinctive scattering features including the radial breathing mode (RBM) and tangential mode (G-band) [93, 102]. Both RBM and G-band of CNTs are sharp and strong peaks, which can be easily distinguished from autofluorescence of tissue samples, Recently, Raman micro‐ spectroscopy of SWNTs has been applied for imaging of tissue samples, live cells, and small animal models [96-98]. Tumor targeted delivery by RGD peptide functionalized SWNTs has been investigated in murine tumor model [85]. Raman spectroscopy image of excised tissues confirmed efficient targeting of αv*β*3 integrin positive U87MG tumor by RGD [85]. This study also disclosed that CNTs have relatively long circulation time, and rapid renal clear‐ ance, which makes SWNTs an attractive diagnostic and therapeutic delivery vehicle. Zevale‐ ta et al further developed a Raman microscope capable of noninvasive in-vivo evaluation of tumors in mice with RGD-labeled SWNTs. Using the dynamic Raman microscope, pharma‐ cokinetics of SWNTs in the tumor was evaluated immediately following an intravenous in‐ jection of SWNTs. Raman spectral analysis revealed effectiveness of the RGD nanotubes to the integrin expressing U87MG tumor. The noninvasive Raman imaging results were com‐ pared with excised tissues and shows consistency [97].

#### **4.3. Photoacaustic imaging**

**4. Carbon nanotubes for biomedical imaging and detection**

298 Syntheses and Applications of Carbon Nanotubes and Their Composites

photoacoustic imaging [96-98].

**4.1. Photoluminescence imaging**

**4.2. Raman shift imaging**

The well-ordered molecular structure attributes CNTs with multiple distinct optical proper‐ ties, include strong NIR absorption, photoluminescence and Raman shift [92]. Structurally, SWNTs can be viewed as a cylinder rolled up by one layer of graphene sheet. TThere are infinite numbers of ways to roll a graphene sheet into a cylinder. Depending on different ways of wrapping, the particular nanotube could be metallic or semi-conductive. Individual semi-conductive SWNTs with appropriate chirality can generate a small band gap fluores‐ cence of 1 eV, which corresponds to NIR range (900-1600 nm), where biological tissues have very low absorption, scattering, and autofluorescence, and therefore, are very useful for bio‐ logical imaging. In the other side, the inherent graphene structure provides SWNTs with specific Raman scattering signature [93], which is strong enough for use in-vivo imaging. All these optical properties offer opportunities for SWNTs as contrast agents for near-infrared (NIR) photoluminescence imaging [94, 95], Raman imaging and optical absorption agent for

NIR photoluminescence of micelle encapsulated SWNTs was firstly discovered by O.Connel et al [7]. The single-particle dispersion of individual nanotubes was prepared by ultrasoni‐ cally agitating of raw SWNTs in SDS. The tube bundles, ropes, and residual catalyst were removed by ultracentrifugation, since the aggregation of nanotubes would quench fluores‐ cence. One advantage of the photoluminescence of SWNTs over organic fluorescence dyes is that SWNTs have no apparent photobleaching, and therefore, the SWNTs could be a power‐ ful tool for tracking changes in living system. Researchers have applied NIR photolumines‐ cence of SWNTs for tracking endocytosis and exocytosis of SWNTs in NIH-3T3 cells in real time [94, 95]. Moreover, conjugation of antibodies to SWNTs surface allowed specific cell targeting. They have shown that, with conjugation of anti-CD20, SWNTs selectively recog‐ nized CD20 cell surface receptor on B-cells with little binding to receptor negative T-cells. Similarly, with conjugation of Herceptin, SWNTs only recognize HER2/neu positive breast cancer cells. The selective binding of SWNTs was detected by intrinsic NIR photolumines‐ cence of nanotubes. This technique allows deep tissue penetration and high-resolution intra‐

Raman spectroscopy is a sensitive analytical tool for biological samples. It also has advan‐ tages of resistance to autofluorescence and photobleaching, high spatial resolution, and small sample size [101]. CNTs exhibit strong resonance Raman scattering with several dis‐ tinctive scattering features including the radial breathing mode (RBM) and tangential mode (G-band) [93, 102]. Both RBM and G-band of CNTs are sharp and strong peaks, which can be easily distinguished from autofluorescence of tissue samples, Recently, Raman micro‐ spectroscopy of SWNTs has been applied for imaging of tissue samples, live cells, and small animal models [96-98]. Tumor targeted delivery by RGD peptide functionalized SWNTs has

vital microscopy imaging of tumor vessels beneath thick skin [99, 100].

Photoacoustic imaging is an optical imaging technique that combines high optical absorp‐ tion contrast with diffraction-limited resolution of ultrasonic imaging, which allows deeper tissues to be viewed in living subjects. In photoacoustic imaging, short pulses of stimulating radiation are absorbed by tissues, resulting a subsequent thermal expansion and ultrasonic emission that can be detected by highly sensitive piezoelectric devices. However, many dis‐ eases, for example cancer, in their early stages, do not exhibit a natural photoacoustic con‐ trast, therefore administering an external photoacoustic contrast agent is necessary. Owing to the strong light absorption characteristic [77], the CNTs can be utilized as photoacoustic contrast agents. De la Zerdaet. al, in the first time, applied SWNTs for in-vivo imaging of tumors in mice [103]. In this study, SWNTs was surface modified by PL-PEG and further conjugated with cyclic RGD peptides for targeting αvβ3 integrin on cancer cells. Intrave‐ nous injection of these cyclic RGD functionalized CNTs to mice bearing tumors showed eight times stronger photoacoustic signal in the tumors than mice injected with non-targeted CNTs. This study suggested that photoacoustic imaging using targeted SWNTs could con‐ tribute to non-invasive in-vivo cancer imaging [103]. Similarly, in another study, SWNTs functionalized with antibody against αvβ3 integrin for photoacoustic imaging of human glioblastoma tumors in nude mice [104].
