Multifunctional Carbon-Based Nanomaterials: Applications in Biomolecular Imaging and Therapy

Abstract

Molecular imaging has been widely used not only as an important detection technology in the field of medical imaging for cancer diagnosis but also as a theranostic approach for cancer in recent years. Multifunctional carbon-based nanomaterials (MCBNs), characterized by unparalleled optical, electronic, and thermal properties, have attracted increasing interest and demonstrably hold the greatest promise in biomolecular imaging and therapy. As such, it should come as no surprise that MCBNs have already revealed a great deal of potential applications in biomedical areas, such as bioimaging, drug delivery, and tumor therapy. Carbon nanomaterials can be categorized as graphene, single-walled carbon nanotubes, mesoporous carbon, nanodiamonds, fullerenes, or carbon dots on the basis of their morphologies. In this article, reports of the use of MCBNs in various chemical conjugation/functionalization strategies, focusing on their applications in cancer molecular imaging and imaging-guided therapy, will be comprehensively summarized. MCBNs show the possibility to serve as optimal candidates for precise cancer biotheranostics.

1. Introduction

Molecular imaging plays a crucial role in early accurate diagnosis and therapy for 21st-century cancer management.¹ Recently, inorganic nanomaterials (gold nanoparticles, iron oxide nanoparticles, silica nanoparticles, carbon nanoparticles, etc.) with various unique intrinsic physical properties have attracted tremendous interest in cancer theranostic (i.e., combined diagnostic and therapeutic) applications.²⁻⁶ To this end, carbon-based nanoparticles have shown great promise as the next generation of theranostic probes due to their excellent mechanical, thermal, and optical properties (Figure 1).^7,8 Carbon nanoparticles are mainly categorized as graphene, single-walled carbon nanotubes, mesoporous carbon, nanodiamonds, fullerenes, or carbon dots.^9,10

Figure 1.

Schematic illustration of theranostic applications of carbon nanomaterials in cancer.

1.1. Graphene

Graphene family nanomaterials (GFNs), without question, are the most extensively studied materials by virtue of their great number of extraordinary physicochemical properties.^7,11,12 As the world’s strongest, thinnest, and stiffest materials,¹¹ GFNs are characterized by alluring optical characteristics (e.g., the ability to quench fluorescence),¹³ a large specific planar surface area (2630 m² g^–1),¹⁴ and unparalleled thermal conductivity (5000 W m^–1 K^–1).¹⁵ GFNs are distinguished mainly on the basis of various chemical modifications and include graphene, graphene oxide (GO), and reduced graphene oxide (rGO) (Figure 2).^7,16,17 Wherein defect-free graphene is not commonly used due to the difficulty of its bulk synthesis, it can be suspended in solutions and isolated in the gas phase on account of its highly reactive surface, so GO and rGO are widely applied in biomedical areas. GO, a highly oxidized form of chemically modified graphene, has many distinct characteristics resulting from the presence of numerous oxygen-containing hydroxyl groups, carboxylic acids, and epoxides in the plane.^7,18 In terms of its benefits, GO possesses colloidal stability and allows surface modification.^9,19 Moreover, with the presence of the free surface π electrons from unoxidized areas of graphene enabling hydrophobic properties, drug loading by π–π interactions, and non-covalent functionalization, GO can serve as a surfactant to stabilize hydrophobic molecules in solution.²⁰ Recent research has demonstrated that GO can be functionalized by biomolecules (e.g., proteins, peptides, small organic molecules, etc.) to enhance unique physicochemical properties required for specific applications when compared to the use of either material alone. For example, Zhang et al. functionalized GO with folic acid (FA) as a cancer-targeting molecule and loaded doxorubicin (DOX) and camptothecin onto the large surface area of GO via π–π stacking.²¹ Compared with GO, rGO, characterized by lower oxygen content, weaker surface charge, and poorer hydrophilicity, is capable of higher light absorption in the visible and infrared regions.^17,22−24 This is because the chemical reduction restored part of the disrupted π conjugation from highly oxidized GO.¹⁷ As a result, functionalized graphene composite nanomaterials are powerful driving forces in the field of bioapplications.

Figure 2.

(a) AFM image of as-made graphene oxide (GO). (b) AFM of covalently PEGylated nanographene oxide (nano-GO). (c) AFM of reduced nanographene oxide (nano-rGO). All images are on a 10 nm height scale. (d–f) Schematic drawings of (d) as-made GO, (e) nano-GO, and (f) nano-rGO illustrating the reaction steps. Reprinted with permission from ref (17). Copyright 2011 American Chemical Society.

1.2. Single-Walled Carbon Nanotubes (SWNTs)

Since the discovery of carbon nanotubes by Iijima in 1991,²⁵ they have been rapidly developed as a platform for biomedical applications.²⁶ SWNTs are cylindrical and sp²-hybridized carbon nanomaterials with nanometer-scale diameters 100–1000 times less than their length, resulting in very large aspect ratios.^6,27 Owing to their unique structure, SWNTs exhibit intrinsic optical properties (i.e., strong resonant Raman scattering²⁸ and photoluminance in the near-infrared (NIR) range)^29,30 that have potential for implementation in bioimaging.³¹⁻³⁵ However, composites incorporating SWNTs have not yet been proven to be competent theranostic agents, mostly because of their poor interaction with the surrounding matrices, leading to inefficient loading ratio and instability.⁹ Therefore, many recent research efforts have been focused on functionalization of SWNTs for bioimaging, ranging from single-modal imaging²⁹ to multimodal imaging,³⁶ and even imaging-guided chemo-photothermal therapy.^37,38 By combining these unique and robust nanomaterials, SWNT–nanoparticle hybrid structures give access not only to the individual properties of the nanoparticles but to additional superiorities, such as remarkable biocompatibility and synergistic properties.

1.3. Mesoporous Carbon Nanoparticles (MCNs)

Mesoporous nanoparticles, especially mesoporous silica-based nanoparticles (MSNs), have been applied in the fields of biotechnology and nanomedicine due to their high surface area, large pore volume, and uniform mesoporous structures.³⁹⁻⁴⁴ However, MSNs have severe drawbacks, such as tedious surface modification processes,³⁹ low loading capacity,⁴⁵ and potential toxicity for in vivo applications.⁴⁶⁻⁴⁸ As such, it is continuously a hot topic to construct an intelligent nanoporous platform with simultaneous controlled drug release and diagnostic imaging performance.³⁹ As the newcomers to the carbon-based family of compounds in nanomedicine, three-dimensional mesoporous carbon nanoparticles (MCNs) with pore size in the range 2–50 nm are leading theranostic materials because of their well-defined nanoporous structures, high pore volume, and high biocompatibility.^39,48−53 Specifically, MCNs-based intelligent multifunctional composite nanosystems have gradually emerged at the foreground of bioapplications, especially for serving as intelligent theranostic platforms for stimuli-responsive drug delivery and molecular imaging.^39,49

1.4. Carbon Dots (CDs)

Nanoscale carbon particles (carbon dots, or CDs), a recently discovered class of carbon nanomaterials, have drawn much attention due to their unique properties, such as excellent water solubility and excellent biocompatibility.⁵⁴⁻⁵⁸ Recently, surface-passivated CDs have been reported to exhibit strong photoluminescence (PL), with spectral features and properties in the visible spectrum (Figure 3).^59,60 CDs that exhibit bright photoluminescence in the visible region are characterized by two distinctive features: (1) small size (sub-10 nm) and (2) surface passivation.^59,60 Compared with conventional organic dyes and semiconductor quantum dots (QDs), CDs possess excellent fluorescence properties, such as high photostability, broad excitation spectra, and tunable emission spectra.^59,61,62 Therefore, CDs have received special attention in potential applications such as bioimaging and drug delivery.

Figure 3.

(A) Surface passivation by attaching simple organic species to acid-treated carbon dots. (B) Representative STEM images of carbon dots surface-passivated with PEG1500N. (C) Photograph of the PEG1500N-attached carbon dots solution (a) excited at 400 nm under band-pass filters of different wavelengths as indicated and (b) excited at the indicated wavelengths directly. Reprinted with permission from ref (60). Copyright 2006 American Chemical Society.

1.5. Other Carbon-Based Nanomaterials

Nanodiamonds (NDs), referring to an octahedral architecture ranging from 5 to 50 nm in size, have seen significant progress in their development as promising candidates for molecular theranostic applications⁶³⁻⁶⁵ due to their exceptional biocompatibility, easy functionalization versatility, high adsorption capacity, and thermal properties.^6,66−68 NDs can produce PL with emission in the red and NIR spectral regions (575–750 nm) and do not bring about photoquenching,^69,70 which typically results from nitrogen vacancy color centers embedded in a diamond matrix.^71,72 In addition to fluorescence properties, NDs also give access to magnetic properties because of their facet-specific electrostatics.⁷³ Moreover, NDs can serve as promising drug carriers due to their unique surface electrostatics.⁷⁴ With the above-mentioned electrochemical and optical properties, NDs’ surfaces can be functionalized to achieve synergistic effects with fluorescent molecules,⁷⁵⁻⁷⁷ DNA,⁷⁸ siRNA,⁷⁹ proteins,^80,81 lysozyme,⁸² growth hormone,⁸³ cytochrome c,⁸⁴ alcohol dehydrogenase,⁸⁵ antibodies,^86,87 anti-cancer drugs,^74,88 and dopamine derivatives.^89,90 As such, developing NDs for future theranostic applications has extensive prospects.⁹¹

Fullerenes (C₆₀), soccer-ball-shaped molecules composed of 12 pentagons formed by C₅–C₅ single bonds and 20 hexagons formed by C₅–C₆ double bonds, have been widely studied since their discovery in 1985.⁹²⁻⁹⁵ Subsequently, numerous fullerenes with other carbon numbers (e.g., C₇₀, C₇₆, C₈₀, and so on) have been produced,⁹⁶⁻⁹⁸ wherein the condensed aromatic rings present in the C₆₀ result in an extended π-conjugated system of molecular orbitals which produce significant absorption of visible light.^99,100 With the absorption of visible light and efficient intersystem crossing to a long-lived triplet state, fullerenes can generate reactive oxygen species upon illumination and so give access to photosensitizers (PSs).^101,102 Recently, functionalized C₆₀ has been realized by covalent attachment with various groups (e.g., −OH, −NH₂, −COOH) to make more promising candidates for theranostic applications.^93,101

In this article, we will systematically discuss the above-described carbon-based nanomaterials in terms of their functionalization, characterization, and implementation for a host of theranostic applications. While there are already numerous excellent reviews focusing on carbon-based nanomaterials’ imaging and cargo delivery, the molecular imaging and imaging-guided therapy applications of carbon-based nanomaterials have not been comprehensively reviewed. Moreover, we present the first of effort to combine MCNs with other carbon-based nanomaterials for perfect carbon-based nanomaterials. This will be followed by an in-depth analysis of the various carbon-based nanomaterials that have been developed for theranostic applications.

2. Theranostic Application of Multifunctional Carbon-Based Nanomaterials

2.1. Graphene

2.1.1. Fluorescence Imaging and Therapy

In theoretical and experimental studies, small graphene (less than 10 nm) can be used as QDs, which can be realized by reducing the connectivity of the π-electron network to induce a band gap using physical or chemical methods.¹⁰³ Recently, graphene quantum dots (GQDs) have attracted increasing interest for implementation into bioimaging applications.¹⁰⁴ As has been reported by Zhu et al., GQDs with strong green fluorescence have been synthesized via a one-step solvothermal method for cellular imaging.¹⁰⁵ In addition, GQDs were explored to serve as a new generation of photodynamic therapy (PDT) agents.¹⁰⁶ The new agents overcame the limitations of conventional agents, such as low singlet oxygen quantum yields, photobleaching, and poor biocompatibility. Although GQDs produced a very high singlet O₂ quantum yield, this situation was attained only in the visible light region and can only offer treatment of near-skin tumors but not deeply buried tumors. Given these findings, Kalluru et al. successfully assembled a novel NIR-light-activatable theranostic platform based on GQDs (Figure 4A).¹⁰⁷ It was demonstrated that nano-GO-PEG-FA sensitized the generation of singlet O₂ to mediate bimodal PDT and photothermal therapy (PTT) effects on effective damage to solid tumors even in deep tissues under ultra-low laser doses of NIR light excitation. Moreover, nano-GO-PEG-FA can exhibit wavelength-dependent PL under NIR light excitation, so it is suitable for in vivo deep fluorescence imaging (Figure 4B). In view of these advantages, more GQDs-based theranostic platforms will be widely explored for fluorescence-guided therapy.

Figure 4.

(A) Schematic illustration of the preparation of GO-PEG-folate to facilitate the combination of nanomaterial-mediated photothermal and photodynamic therapeutic destruction of tumors. (B) In vivo fluorescence imaging from PBS (left) and GO-PEG-folate (right) mice at 24 h after intravenous injection. Reprinted from ref (107). Copyright 2016 Elsevier Ltd.

In addition to serving as QDs, graphene can also act as a carrier for other QDs or dyes to reduce their adverse effects and enhance their properties. As such, a great number of studies on dye/QDs-labeled graphene nanocomposites for fluorescence imaging have also been conducted. On one hand, Gao et al. developed a thermo-activatable hybrid nanocomplex, called CPGA, by conjugating a NIR dye (Cy5.5)-labeled matrix metalloproteinase-14 (MMP-14) substrate (CP) onto the GO/Au complex (GA),¹⁰⁸ wherein the Cy5.5 fluorescent signal is quenched by surface plasmon resonance (SPR) capacity from GA, yet it can boost stronger fluorescence signals after separation from GA. Compared to the other activatable fluorescent probes, CPGA had prominent quenching efficiency (95%), which gives it remarkable promise for future development of intelligent nanoplatforms. In another study, Cy5.5-labeled molecular beacon (MB) was conjugated onto GO by π–π stacking interactions for highly sensitive detection and imaging of miRNAs.¹⁰⁹ GO can induce efficient fluorescence quenching of the dyes on MB. When the Cy5.5-MB target molecule reacted with the miRNAs to form a duplex, it was released from the GO surface and restored the fluorescence of one dye molecule in living cells and in vivo. On the other hand, to prevent fluorescence quenching, Hu et al. used a surfactant coating method to synthesize a hybrid nanocomposite of core–shell CdSe/ZnS QDs and rGO.¹¹⁰ In addition to reducing the toxicity of QDs, the nanocomposite also preserved thier fluorescence characteristics by maintaining a suitable spacer between the QDs and rGO (Figure 5). Meanwhile, the QDs also acted as an optical indicator for the PTT effect by degradation resulting from the thermal effect on the irradiated rGO.

Figure 5.

(A) Schematic representation of the synthesis of QD-rGO. (B) Fluorescence of the QD-tagged 38 nm rGO and the QD-tagged 260 nm rGO. (C) Cellular uptake of FA-QD-US-rGO. (D) Schematic illustration of QD-rGO under irradiation that causes cell death and diminished QD fluorescence (a) and images before (b) and after (c) irradiation. Reprinted with permission from ref (110). Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA.

2.1.2. Raman Imaging and Therapy

Raman scattering is a photon scattering process in which photons’ emission wavelengths are shifted under light excitation rather than PL.^111−113 When the laser energy reaches the desired value, required for electron transition from the valence band to the conduction band, the generated signal reflects “resonance Raman scattering”.^114,115 Typically, GO can be characterized by Raman spectroscopy on the basis of its distinctive Raman spectra (D band at 1350 cm^–1 and G band at 1600 cm^–1).¹¹⁶ However, the inherent Raman scattering signals of GO itself are usually weak. Recently, GO was modified with noble metals (e.g., Au and Ag) to promote its Raman intensity and sensitivity, which is referred to as surface-enhanced Raman scattering (SERS).^116,117 Liu et al. decorated Au/GO hybrids that could serve as a flexible Raman probe for bioimaging applications through Raman mapping.¹¹² Compared to the original GO, the Raman intensity of the Au/GO hybrids was remarkably enhanced 4-fold as a consequence of the surface enhancement effect of the AuNPs. Similar results were demonstrated by other researchers.¹¹⁸ Moreover, SERS shows ultrasensitivity and quantitative abilities in monitoring drug release behaviors due to its advantage of narrow spectral bands, which overcome the drawbacks of the fluorescence-traceable nanocarriers for tracking intracellular drug release dynamics.¹¹⁹

2.1.3. Magnetic Resonance Imaging (MRI) and Therapy

MRI contrast agents can reduce the T₁ and/or T₂ relaxation times of protons to enhance the magnetic resonance signal.¹²⁰ However, the conventionally used Gd³⁺-based T₁ contrast agents have some limitations, such as short half-life, low sensitivity, nephrogenic systemic fibrosis, and renal failure.^121,122 In this regard, Gd-based nanostructures have been extensively studied to increase T₁ relaxivities and reduce safety concerns. Two-dimensional graphene serving as a carrier has attracted a great deal of attention owing to its unique physicochemical properties. Zhang et al. developed a theranostic agent based on GO-Gd complexes.¹²³ In this case, diethylenetriaminepentaacetic acid (DTPA) was covalently conjugated with GO, followed by Gd³⁺ complexation, resulting in formation of GO-DTPA-Gd. Next, DOX and either an anti-cancer drug or a fluorescent dye was loaded on the surface of GO via π–π interactions for integrating imaging and intelligent pH-sensitive drug delivery. The agent exhibited much better contrast enhancement than the traditional Magnevist brand MRI contrast agent. Moreover, after being uptaken by cancer cells, the DOX on GO-DTPA-Gd complexes was successfully released to achieve accurate chemotherapy.

In recent years, magnetic nanoparticles (MNPs), such as Fe₂O₃ and Fe₃O₄, have been proven to possess unique magnetic properties offering sufficient sensitivity for T₂-weighted imaging.¹²⁴ In addition, suitable aggregation of MNPs has been reported to increase relaxivity rates (r₂).¹²⁵ However, uncontrolled aggregation of iron oxide nanoparticles (IONPs) can cause precipitation of the nanoparticles and thus reduce their time of circulation in the blood due to reticuloendothelial system (RES) clearance.¹²⁶ Recently, magnetic Fe₃O₄/GO complexes were employed to realize a multifunctional theranostic platform. For example, Wang et al. synthesized a multifunctional nanocarrier system based on magnetic Fe₃O₄/GO nanocomposites via inverse chemical co-precipitation.¹²⁷ This system has difunctional characteristics, with excellent MRI and intelligent pH-sensitive drug delivery.

In order to further optimize the theranostic platform, Wang et al. developed a targeting peptide-modified mesoporous silica with magnetic graphene.¹²⁸ This type of composite possesses many unique properties, such as large T₂ relaxation rates (r₂), facile magnetic separation, high NIR photothermal heating, high DOX loading capacity, and laser-mediated/pH-dependent synergistic release behavior, which make it a robust platform for cancer theranostics.

2.1.4. Photoacoustic (PA) Imaging and Therapy

Photoacoustic (PA) imaging, serving as a biophotonic diagnostic modality, is mainly achieved by a safe thermal expansion mechanism.^129,130 A PA signal is generated within the compounds when they are irradiated by short-time laser pulses.¹³⁰ Although PA imaging overcomes the optical diffusion limit and gives access to deeper tissue imaging by combining optical excitation with ultrasonic detection, its sensitivity, determined by the compounds’ light absorbance and heat capacity, is quite limited.^131−133 Thus, it is necessary to improve the conversion efficiency of light energy to thermal energy in order to achieve sensitive PA imaging. Recently, various PA contrast agents with remarkable optical absorbance properties, such as gold nanoparticles (AuNPs) and gold nanorods (AuNRs), have been widely employed in efforts to enhance PA imaging sensitivity.^134,135 It was demonstrated that the efficiency of heat transfer between heterogeneous materials played a significant role in obtaining amplified PA intensity.¹³⁶ Numerous studies have highlighted graphene-based heterogeneous systems as promising imaging agents for achieving high-resolution PA imaging. For example, Moon and co-workers produced rGO-coated AuNRs with superior amplified PA performance.¹³⁷ In this case, positively charged AuNRs were dropped onto the negatively charged rGO (Figure 6). The improved PA signal amplitude effect can reduce the damage to normal tissues from laser or imaging agents.

Figure 6.

(A) Overall synthesis schemes of GO-AuNRs and rGO-AuNRs. (B) Representative PA images obtained using excitation wavelengths of 700 and 800 nm (input laser power, 6.2 mJ/cm²; pulse rate, 10 Hz). (C) Photoacoustic amplitudes obtained with different laser power inputs. (D) Photoacoustic images of AuNRs, GO-AuNRs, and rGO-AuNRs in live mice. Overlaid ultrasound (gray) and photoacoustic (red) images. Reprinted with permission from ref (137). Copyright 2015 American Chemical Society.

In another study, a new nanocomposite of indocyanine green-loaded polydopamine-rGO (ICG-PDA-rGO) was developed in order to simultaneously obtain an amplified PA signal and imaging-guided therapy effects.¹³⁸ The ICG-PDA-rGO showed not only prominent PA imaging sensitivity but also sufficient photothermal ablation effect for tumor. Overall, these studies provide remarkable evidence for the benefits of combining PA contrast agents with GO sheets to enhance PA intensity.

2.1.5. Radionuclide Imaging and Therapy

Radionuclide imaging (positron emission tomography (PET) and single-photon emission computed tomography (SPECT)), an imaging modality to measure biochemical metabolism, requires radiolabeled imaging agents (e.g., ⁶⁴Cu, ⁶⁶Ga, and ¹²⁵I).^139−141 Numerous molecules, such as antibodies,¹⁴² nucleic acids,¹⁴³ small-molecule ligands,¹⁴⁴ and especially peptides,^145−147 have been labeled with radionuclides as PET probes.¹⁴⁸ Although decent tumor uptake was shown, fast clearance of the radiolabeled molecules from the tumors and short half-lives of the radionuclides limited their application in longitudinal tumor detection and radiation therapy (RT).^149,150

Recently, some studies have been focused on the incorporation of radiolabeled molecules with GO for integration of imaging and therapeutic components. For example, Shi et al. designed ⁶⁴Cu-labeled and antibody-conjugated rGO for tumor vasculature-targeted PET imaging and PTT.¹⁵¹ The resulting ⁶⁴Cu-NOTA-rGO-TRC105 exhibited long-term high aggregation in tumor and favorable PTT effect. In addition, Yang et al. similarly synthesized functionalized rGO with ⁶⁴Cu and antibody for PET early metastasis detection, targeted delivery of chemotherapeutics and PTT.¹⁵² Moreover, Chen et al. developed ¹³¹I-labeled rGO-PEG for SPECT imaging-guided RT and PTT for cancer.¹⁵³

2.1.6. Multimodal Imaging and Therapy

Multimodal imaging has attracted increasing interest to overcome the limitations of single-modal imaging.^154−156 To achieve various modalities’ integration, incorporation of independent modal contrast agents into a flexible platform has gradually become a main strategy.^157,158 However, this way usually requires complicated synthetic procedures and harsh conditions. Graphene is a competent platform due to its unique merits, such as large surface area, remarkable π–π interactions, and electronic properties.^159,160 Xu et al. developed an integrated imaging system (⁶⁴Cu-rGO-IONP-PEG) combining MRI with PET and PA imaging to obtain higher quality imaging and more precise diagnostic data.¹⁶¹ PET with high sensitivity and quantitative analysis, MRI featuring with high spatial resolution, and PA imaging giving access to deep tissue detection are excellent complements for disease diagnosis. In a follow-up report, photosensitizer was loaded onto nanographene via π–π stacking for multimodal-imaging-guided synergistic therapy. The resulting GO-PEG-[⁶⁴Cu]HPPH achieved not only PET, PA, and fluorescence imaging simultaneously but also trimodal-guided tumor PDT.¹⁶² In addition, Jin et al. developed poly(lactic acid) (PLA) microcapsules encapsulating GO and AuNPs for tumor PTT guided by ultrasound (US)/computed tomography (CT) bimodal imaging.¹⁶³ The PLA microcapsule can enhance US imaging. AuNPs with X-ray attenuation properties enable the use of CT contrast agents or radiosensitizers.^164−166 Meanwhile, AuNPs can enhance the effect of GO serving as a NIR-light absorbing agent, which efficiently converts absorbed light into heat. These studies give powerful evidence that graphene can serve as a multimodal imaging platform for tumor theranostic applications and can, to some extent, overcome the limitations of other platforms.

2.2. Single-Walled Carbon Nanotubes (SWNTs)

2.2.1. Fluorescence Imaging and Therapy

The inherent chirality-characteristic PL of semiconducting SWNTs is characterized by NIR-I region excitation, large Stokes shift, ultralow autofluorescence background, and deep tissue penetration.^167−170 PL of SWNTs occurs when electron–hole (e-h) pairs recombine to emit a photon.^171,172 Some studies have been reported about PL of SWNTs in micelle solutions, air, and polymers.¹⁷³ However, the relatively low quantum yield (QY) of SWNTs (typically 0.1%–1.5%) severely restricted its further bioapplications.^174,175 It is well documented that the localized surface plasmon resonance (LSPR) of noble metals (e.g., gold or silver) enables surface-enhanced fluorescence (SEF).^176,177 The space between the carrier and the precious metal surface has a close relationship with SEF that is proportional to QY.^178−180 One of the key approaches to enhance the fluorescence as well as depress the quenching effect is to find a good linker that can closely connect noble metals to the surface of the SWNTs yet protect the SWNTs from direct conjugation. For this, Yang and co-workers attached DNA to the surface of SWNTs via π–π stacking interactions and then assembled AuNPs onto the DNA-SWNTs, resulting in formation of AuNP-DNA-SWNT hybrids (Figure 7).²⁹ The DNA, serving as a spacer, separated AuNPs from SWNTs to achieve excellent SEF from the SWNTs. Compared with the SWNTs, the PL of AuNP-DNA-SWNTs was greatly more enhanced. In addition to the above method of locating noble metals on the SWNTs surface, PEGylated phospholipids were also used to enhance intrinsic PL and strengthen optical absorbance in the NIR.¹⁸¹ The modified SWNTs were characterized by not only superior PL imaging but also a greater PTT effect compared to AuNRs. In a similar study, SWNTs were sonicated with sodium cholate, followed by surfactant DSPE-mPEG_5k exchange, to form phospholipid–polyethylene glycol (PEG)-coated SWNTs (referred to as exchange-SWNTs) for NIR imaging.¹⁸² The NIR PL of exchange-SWNTs is better than that of direct SWNT/DSPE-mPEG_5k conjugates. Overall, despite the relatively low quantum yield (QY) of SWNTs, the functionalized SWNTs with inorganic materials (e.g., noble metal Au and Ag) or organic material (e.g., PEG and DNA) can show greatly enhanced PL and have superior prospects in NIR PL bioimaging.

Figure 7.

(A) Schematic of PL enhancement of the recognition DNA-SWNT pair and the non-recognition DNA-SWNT pair. (B) PL emission spectra upon excitation at 650 nm of AuNP-(ATT)₄AT-SWNTs with addition of concentrated AuNPs from 0 to 34 μL. (C) PL integral area of all peaks (red dots) and PL peak area percentage in (B). PLE maps of (D) the original (ATT)₄AT-SWNTs solution, (E) solution with addition of 4 μL of AuNPs, and (F) solution with addition of 29 μL of AuNPs. PL intensities for (D)–(F) follow the same color scale. Reprinted with permission from ref (29). Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA.

2.2.2. Raman Imaging and Therapy

SWNTs with typical Raman spectra can give access to Raman imaging.¹⁸³ The characteristic structure of SWNTs can produce sharp van Hove singularities under the electronic density of states (eDOS), which results in the occurrence of resonant Raman scattering.¹⁸⁴ Compared with graphene, SWNTs themselves can produce stronger characteristic resonance-enhanced Raman peaks with tangential G bands (∼1580 cm^–1) away from the excitation wavelength, which contributes to sensitive detection of unmodified SWNTs by Raman microspectroscopy.^113,185 Heller et al. first reported the intrinsic Raman properties of SWNTs for live cell imaging.¹⁸⁶ In their work, DNA-modified SWNTs (DNA-SWNTs) were used as cell markers that retained persistent Raman scattering in live cells, which allowed them to be used for long-term tracking in biological samples over months. Liu et al. investigated the biodistribution of functionalized SWNTs by invasive Raman spectroscopy of excised tissues to confirm the targeting effect.¹⁸⁷ Zavaleta et al. analyzed the targeting effect of arginine-glycine-aspartic acid (RGD) peptide-modified SWNTs by non-invasive Raman imaging in vivo.²⁸ Although SWNTs exhibit the appropriate inherent Raman signals, too much time is required to get a satisfactory Raman image.¹⁸³ It was hoped that SERS might overcome this obstacle. As such, Wang et al. developed a folic acid (FA)-conjugated SWNT-Au nanocomposite (SWNT-Au-PEG-FA) for PTT and SERS imaging whose imaging time was remarkably shortened compared to that of targeted SWNTs (Figure 8).¹⁸⁸

Figure 8.

(A) Schematic illustration of the synthesis of SWNT–metal nanocomposites. (B) Representative TEM images of the SWNT-Au-PEG nanocomposite. (C) Raman imaging and averaged Raman spectra of cells. (a–h) Raman images of SWNT-Au-PEG-FA (a,b), SWNT-Au-PEG (c,d), SWNT-PEG-FA (e,f), and SWNT-PEG (g,h) labeled KB (a,c,e,g) and HeLa (b,d,f,h) cells. Reprinted with permission from ref (188). Copyright 2012 American Chemical Society.

2.2.3. Magnetic Resonance Imaging and Therapy

In addition to graphene, SWNT-based nanomaterials were also used as scaffolds to anchor MNPs or Gd³⁺-based agents in order to improve the imaging quality. MNPs/Gd³⁺ clustering on SWNTs sheets would not only enhance MRI contrast but also improve the physiological stability of the aggregated MNPs/Gd³⁺.^189,190 To this end, Sitharaman et al. developed ultra-short SWNTs (US-tubes) with nanoscale loading and confinement of Gd³⁺_n clusters to enhance longitudinal relaxivity, r₁.¹⁹¹ The MRI efficacies of these Gd³⁺_n@US-tubes were 40–90 times larger than those of clinical used Gd³⁺-based contrast agents. Safety development of Gd³⁺n@US-tube as contrast agents was later demonstrated by Holt et al.¹⁹² Further advancing upon these hybrid platforms, Gd³⁺-SWNTs were confined inside the nanoporous structure of silicon particles (SiMPs) by Ananta et al. to improve nanoscale confinement for enhancement of the image contrast.¹⁹³ The r₁ relaxivity of the resulting SiMPs/Gd-SWNTs composites increased by approximately 1.5 times compared to that of Gd-SWNTs (Figure 9). The enhancement in contrast can be attributed to the geometrical confinement of the agents, which largely increased their tumbling time (τ_R). Therefore, nanoscale confinement could serve as a new and general strategy to enhance the contrast of MNPs or Gd³⁺-based contrast agents.

Figure 9.

New MRI nanoconstructs. Schematic showing Magnevist (A), gadofullerenes (GFs) (B), and debundled gadonanotubes (GNTs) (C). Scanning electron micrographs of (D) quasi-hemispherical and (E) discoidal particles. (F) Cartoons showing Magnevist, GFs, and GNTs (left to right) entrapped within the porous structure of the SiMPs. Reprinted with permission from ref (193). Copyright 2010 Nature Publishing Group.

Recently, to integrate imaging and therapy function, Hou et al. developed a targeted multifunctional theranostic system based on Gd³⁺-SWNTs. Herein, hyaluronic acid (HA)-modified SWNTs (SWNTs-HA) were first synthesized, and then DOX was conjugated with HA by disulfide bonds (SWNTs-HA-ss-DOX). Finally, Gd³⁺ ions were loaded on the sidewall defects of SWNTs.³⁷ When Gd/SWNTs-HA-ss-DOX were uptaken into cancer cells, high concentrations of glutathione (GSH) broke the disulfide bond, which resulted in DOX release to achieve precise chemotherapy.

In addition to the above studies, mainly referring to T₁-weighted images, T₂ negative contrast agents based on MNPs-SWNTs have also been widely studied.³⁶ Recently, Al Faraj et al. employed an iron-tagging method to successfully develop iron-tagged and antibody-conjugated SWNTs for enhanced magnetic targeting and non-invasive MRI monitoring.¹⁹⁴ Therefore, perhaps Gd-based T₁ or MNPs-based T₂ contrast agents conjugated with SWNTs could reach great imaging effects even for better imaging and treatment.

2.2.4. Radionuclide Imaging and Therapy

Similar to graphene, radiolabeled SWNTs conjugates also have been explored. Liu et al. reported ⁶⁴Cu-radiolabeled SWNTs functionalized with phospholipid-PEG (PL-PEG₅₄₀₀) and RGD peptide (Figure 10).¹⁸⁷ PEGylation imparted to SWNTs high hydrophilicity and resistance to protein non-specific binding, which resulted in reduced RES uptake, long blood circulation, and stable PET imaging of ⁶⁴Cu-labeled SWNT-PEG-RGD. In addition, Cisneros and co-workers successfully loaded ⁶⁴Cu²⁺ ions into SWNTs platform by sonication without the use of chelating ligands.¹⁹⁵ The new strategy achieved stable loading of contrast agents. The resulting ⁶⁴Cu@GNTs give access to excellent imaging quality.

Figure 10.

(A) Schematic drawings of non-covalently functionalized SWNT-PEG2000, SWNT-PEG5400, SWNT-PEG2000-RGD, and SWNT-PEG5400-RGD with DOTA-⁶⁴Cu. (B) MicroPET images of two mice at various time points post tail-vein injection of ⁶⁴Cu-labeled SWNT-PEG2000 and SWNT-PEG5400, respectively. (C) MicroPET images of mice. Reprinted with permission from ref (187). Copyright 2007 Nature Publishing Group.

2.2.5. Multimodal Imaging and Therapy

To sum up, semiconducting SWNTs, characterized by remarkable resonance-enhanced Raman scattering, alluring PL, and strong optical absorption in the NIR region, could be competent multimodal imaging platforms. Given this, Wang et al. designed targeted SWNTs with attached Fe/Co nanoparticles to label human mesenchymal stem cells (hMSCs).¹⁹⁶ The multimodal imaging platform gives access to triple-modal imaging, including strong Raman signals, T₂-weighted MR images, and intense PA signals. These functionalized SWNTs were characterized by high chemical stabilities and photostabilities. This study highlighted the potential of well-functionalized SWNTs as multimodal imaging probes. For further therapy under multimodal imaging guidance, Zhao et al. prepared Mn²⁺-chelated and ³¹I-radiolabeled SWNTs as a theranostic platform for tumor multimodal imaging and therapy.¹⁹⁷ Their previous work confirmed that PDA with phenol groups could be efficiently labeled with radioactive iodine (e.g., ¹³¹I) using the chloramine-T oxidation method. PDA-modified SWNTs were labeled with ¹³¹I using the same method used for tumor gamma imaging. Depending on the inherent strong NIR absorbance of SWNTs, ¹³¹I-based radioactivity and T₁/T₂ MRI of Mn²⁺, the functionalized SWNTs achieved magnetic resonance and gamma bimodal imaging-guided PTT and radioisotope therapy under NIR.

2.3. Imaging and Therapy-Based Mesoporous Carbon

Three-dimensional MCNs with high drug loading and controlled drug release capability are highly desirable to establish an intelligent stimuli-responsive cancer theranostic system which can reduce adverse effects of cytotoxic drugs and achieve treatment monitoring.¹⁹⁸ Recently, MCNs combining both imaging contrast agents and drugs simultaneously have received much attention. Zhou et al. reported DOX-loaded colloidal mesoporous carbon nanospheres (Meso-CNs) for tumor PTT and PA imaging.¹⁹⁹ By side-by-side comparisons with SWNTs, graphene, and AuNRs, the Meso-CNs were shown to possess higher absorption coefficients than those of CNTs and graphene, and even comparable with those of AuNRs. Moreover, Zhang et al. synthesized DOX-loaded hollow mesoporous silicon/carbon (Si/C) NPs (PEG-Si/C-DOX) for highly effective PA imaging-guided chemo-thermal tumor therapy.²⁰⁰ Zhang et al. constructed a highly efficient multiple-stimuli-responsive nanosystems based on hollow MCNs (HMCNs) for pH-responsive MRI and on-demand drug release (Figure 11).³⁹ Manganese oxide (MnOx) NPs can be in situ generated and incorporated into the framework or onto the surface of HMCNs to form MnOx-HMCNs by a redox reaction between KMnO₄ and a carbonaceous framework of HMCNs. The “OFF” state that kept Mn atoms restricted within the MnOx NPs can been broken up by the acidic microenvironment in tumor cells to release Mn²⁺ ions, which result in significantly enhanced T₁ performance (“ON” state). Importantly, DOX loaded onto MnOx-HMCNs through π–π stacking was also highly sensitive to pH variations. In addition, introduction of high-intensity focused ultrasound could significantly accelerate the release rate of DOX from MnOx-HMCNs.

Figure 11.

Schematic illustration of (A) synthetic procedure for MnOx-HMCNs, (B) incorporation of MnOx NPs into the framework of HMCNs, and (C) cellular uptake and dual pH-responsiveness of MnOx-HMCNs for T₁-weighted MRI and anti-cancer drug release. (D) (a) SEM image of RBCs-shaped HMCNs and (b) TEM images of MnOx-HMCNs with low magnification. (E) Plot of T₁^–1 versus Mn concentration of MnOx-HMCNs. (F) HIFU-triggered DOX release percentage under different irradiation parameters. Reprinted from ref (49) with permission from The Royal Society of Chemistry.

Furthermore, a GQDs-based fluorescent mesoporous carbon nanoshell (FMP-CNS) was developed as a hydrophobic drug carrier that integrated functions of stimuli-responsive drug release, multimodal optical imaging, and photothermal therapy/chemotherapy.⁴⁸ The FMP-CNS can serve as an optical marker for confocal, two-photon, and NIR fluorescence imaging. Moreover, the hydrophobic drug paclitaxel (PTX), loaded on the FMP-CNS, can be controllably released under NIR irradiation. MCNs can also carry fluorescent dyes. In a recent study, a fluorescent probe based on oxidized MCNs (OMCNs)was constructed, wherein Cy3-labeled ssDNA (P₀ aptamer) was loaded on the surface of OMCNs by π–π stacking, resulting in almost complete fluorescence quenching (Figure 12).⁴⁹ The P₀ aptamer can specifically bind to the cell-surface mucin1 (MUC1) marker which is overexpressed in many malignant tumors (e.g., breast cancer and prostate cancer). Once exposed to MUC1, P₀ aptamer, separated from OMCN, decomposed to achieve fluorescence recovery.

Figure 12.

(A) Schematic illustration of the sensing principle based on the OMCN/P₀-Cy3 aptasensor. (B) MUC1-responsive fluorescence recovery in the buffer solution recorded by fluorescence spectrophotometry. (C–E) Sensing performance of the OMCN/P0-Cy3 aptasensor at cell level (confocal fluorescence microscopy image), tissue level (inverted fluorescence microscopy image), and whole animal level (in vivo fluorescent image), respectively. Reprinted with permission from ref (49). Copyright 2015 American Chemical Society.

2.4. Imaging and Therapy-Based Carbon Dots

At present, CDs are receiving considerable attention for potential theranostic applications owing to their unique superior properties, such as inherent favorable aqueous solubility, good photostability, easy modification, low toxicity, and excellent biocompatibility.^201−206 For example, Cao et al. demonstrated the potential of CDs for cell imaging with two-photon excitation in the NIR.⁵⁹ Later, Yang et al. reported the first use of CDs for optical imaging in vivo.²⁰⁷ Recently, a new type of fluorescent carbon dots (CD-Asp), with the ability to specifically target glioma, were successfully synthesized via a simple thermolysis route using d-glucose and l-aspartic acid as starting materials.²⁰⁸ CD-Asp could act as a targeting fluorescence imaging agent for non-invasive glioma diagnosis, and has the potential for use in constructing an intelligent nanomedicine integrating diagnostic, targeting, and therapeutic functions. In addition, there have also been some studies about the use of functionalized CDs for multimodal imaging. Zhang et al. employed a hydrothermal carbonization (HTC) approach for facile preparation of iodine-doped carbon dots (I-doped CDs).²⁰⁹ The resulting CD composites exhibited not only favorable PL but also high X-ray attenuation. In another study, targeted magnetic mesoporous silica nanoparticles (Fe₃O₄@mSiO₂) with fluorescent CDs were constructed for fluorescence imaging and MRI (Figure 13).²¹⁰

Figure 13.

(A) Schematic route for the synthesis of Fe₃O₄@mSiO₂-TPP-CDs nanoplatform. (B) Differential interference contrast and confocal laser scanning microscopy images of HeLa cells exposed to 100 μg/mL of Fe₃O₄@mSiO₂-TPP/CDs. Reprinted with permission from ref (210). Copyright 2015 American Chemical Society.

However, the complex synthetic route and high cost of CDs greatly limit their practical applications. To address this issue, a highly promising approach for the synthesis of carbon materials is based on the use of well-defined molecular precursors.²¹¹ Ge et al. prepared novel CDs using polythiophene phenylpropionic acid (PPA) as the precursor for fluorescence/PA imaging-guided cancer PTT.²¹² Li et al. successfully synthesized tetraphenylporphyrin-based carbon dots (TPP CDs) for PDT of hepatoma.²¹³ The TPP CDs can efficiently generate ¹O₂, resulting in significant cancer PDT efficacy. Jia et al. employed Hypocrella bambusae (HB) as a raw carbon source to successfully synthesize H. bambusae CDs (HBCDs) which could be served as theranostic agents for bimodal fluorescence/PA imaging-guided cancer synergistic PDT/PTT (Figure14).²¹⁴ The obtained HBCDs showed good water solubility, broad absorption, red-light emission, and excellent biocompatibility. Similarly, another type of CDs with intrinsic theranostic properties were prepared by using polythiophenebenzoic acid (PBA) as carbon source for imaging-guided PDT/PTT.²¹⁵ The novel CDs from PBA exhibited properties similar to those of the above HBCDs. Jia et al. designed multifunctionalized CDs on the surface of SiO₂-coated AuNRs (AuNR@SiO₂-CDs) as phototheranostics for cancer PDT/PTT synergistic therapy under the guidance of FL and PA imaging.⁵⁴

Figure 14.

Schematic of synthesis of HBCDs derived from Hypocrella bambusae for tumor bimodal FL/PA imaging-guided synergistic PDT/PTT. Reprinted with permission from ref (214). Copyright 2018 Elsevier B.V.

2.5. Imaging and Therapy-Based Other Carbon Nanomaterials

Nanodiamonds (NDs) can emit significant fluorescence and present perfect photostability with no photobleaching, making them useful for bioimaging without further functionalization.^72,216 Massive research efforts have been devoted to the utilization of NDs as fluorescent markers for cellular tracking in vivo/in vitro.^69,72,217 In addition, NDs are characterized by facet-specific electrostatic properties which contribute to coordination of water molecules on specific facets. Thus, NDs themselves, to an extent, improve the effect of MRI contrast agents. In one study, Hou et al. conjugated Gd to a NDs surface to acquire T₁/T₂ dual-mode MR imaging.²¹⁸ Furthermore, Waddington et al. demonstrated that the nuclear Overhauser effect, a proton–electron polarization transfer technique, enables high-contrast MRI of unfunctionalized NDs in water.²¹⁹ The technique significantly expanded the theranostic capabilities of NDs and opened the possibility that NDs themselves can serve as MR contrast agents. Recent in vivo studies reported the clinical potential of utilizing NDs to deliver chemotherapeutic agents.

Due to their surface-dependent PL, excellent photostability, and biocompatibility, fullerenes can be used for fluorescence imaging.²²⁰ For example, hyaluronated C₆₀ fullerene with a strong intrinsic NIR fluorescence emission was designed for both fluorescence imaging and PDT of HCT-116 tumors (CD44+).²²¹ In addition, Tan and co-workers presented a photoluminescent fullerene-capped MSN system for bioimaging and pH-responsive drug delivery.²²² Recently, metallofullerenes containing Gd complexes (e.g., Gd@C₆₀, Gd@C₈₂, and Gd₃N@C₈₀) have been employed as MR contrast agents with much higher relaxivity and safety than most commercial Gd-chelator agents.^223−225 Gd-ion-associated toxicity is decreased by confining Gd ions inside a robust carbon cage to suppress their leakage. Therefore, the development of Gd-containing metallofullerene contrast agents is highly desirable for multimodal imaging. Han et al. synthesized a high-relaxivity-targeted Gd₃N@C₈₀ contrast agent (ZD2-Gd₃N@C₈₀) by conjugating a small targeting peptide, ZD2 (Cys-Thr-Val-Arg-Thr-Ser-Ala-Asp), for sensitive molecular MRI of breast cancer (Figure 15).²²⁶ Wang et al. developed radionuclide ⁶⁴Cu-labeled DOX-loaded PDA-gadolinium-metallofullerene (Gd₃N@C₈₀, denoted as CDPGM) core–satellite nanotheranostics for multimodal imaging-guided combination cancer therapy.²²⁷ Besides T₁ MRI contrast agents, iron oxide (Fe₃O₄) nanoparticles and upconversion nanophosphors were also loaded onto C₆₀ to achieve effective therapy and live surveillance of tumors.²²⁸ Later, FA-modified C₆₀-IONP-PEG was designed to achieve targeted PDT, release and thermal therapy (RTT), and T₂ MRI.¹⁰¹ Moreover, FA-modified C₆₀@Au hybrid aggregates were synthesized for radiofrequency-based controlled RTT, PDT synergetic therapy, and X-ray imaging.²²⁹

Figure 15.

(A) Schematic illustration of ZD2-Gd₃N@C₈₀ probes. (B) Illustration of tumor targeting with the probes for detection of breast cancer. Reprinted with permission from ref (226). Copyright 2017 Nature Publishing Group, http://creativecommons.org/licenses/by/4.0/).

3. Challenges and Future Prospective

CBNs, as new stars in nanoresearch, have attracted increasing attention in the past few years in a wide array of fields, especially for biological applications such as bioimaging, drug delivery, and cancer therapeutics. In this article, we look at various applications of MCBNs in tumor molecular bioimaging and imaging-guided molecular-level cancer therapy. As a primary conclusion based on the considerable research over the past two decades, it can be said that MCBNs have tremendous potential in early comprehensive diagnosis and precise treatment of cancer due to their intrinsic physicochemical properties.

Although molecular imaging has demonstrated promising potential for MCBNs and will play a significant role pertaining to increased theranostic accuracy as well as improved prognosis of disease, the most realistic direction for highly sensitive and non-invasive theranostic technology is its application to patient care. This means that there are considerable challenges to be addressed before MCBNs can be translated into actual use in clinical practice. Although various functionalization strategies have been developed to reduce toxicity and improve biocompatibility, efforts are still needed to further implement more systematic toxicology studies to determine the interactions between MCBNs and physiological systems and the pharmacokinetics of MCBNs according to U.S. FDA guidelines.

In order to reduce undesirable side effects on the human body, “precision medicine”, which involves accurate localization and precise removal of tumors under imaging guidance, has become the current cutting-edge topic. Some researches have made use of tumor’s microenvironment to active targeting, based on, e.g., low pH, high H₂O₂, and abundant GSH. On the other hand, by controlling the size of the nanomaterials in order to avoid liver and spleen phagocytosis, some researches have optimized the permeability and retention effects of MCBNs to notably improve passive targeting. Although these are exciting strategies, a significant concern that remains is how to precisely target tumor sites to achieve accurate diagnoses and therapy.

For cancer theranostics, image-guided synergistic therapy and intelligent drug delivery systems are the most promising trends. In order to acquire more comprehensive disease information, which is essential for diagnosis and treatment, multimodal imaging is practically feasible for overcoming the inherent limitations of each modality alone. MCBNs can serve as a multiplatform, gathering different imaging agents and therapeutic drugs for cancer diagnosis and therapy integration. Despite the certain challenges and unresolved obstacles, MCBNs have potential clinical advantages as imaging and treatment agents and will be more likely to be applied in clinical settings in the near future. Finally, significant investigation will be required to further develop advanced carbonaceous nanomaterials for improving the efficacy of theranostics and reduce side effects. Once MCBNs are established in that field, their applications in theranostics would become more realistic.

Author Contributions

^# Yanyan Zhang, Minghao Wu, and Mingjie Wu contributed equally to this work.

The authors declare no competing financial interest.