Covalent functionalization

In stark contrast to non-covalent wrapping of SWCNTs, which besides ultrasonic treat-ment is considered a very mild procedure, covalent functionalization by design alters the nanotube’s structure and with it its unique photophysical and electronic properties. As mentioned above, carbon nanotubes can be imagined as a rolled-up sheet of graphene resulting in a susceptibility towards chemical reactions borrowed from large aromatic systems influenced by a certain degree of additional curvature. This curvature results in deviations from bond angles normally found for extended ⇡-systems as well as ring strain and structural defects compensating for that strain. These properties build the foundation for the covalent chemistry of SWCNTs. As a consequence, the large variety of reactions already known for aromatic compounds or graphite represents a toolbox for chemists and material scientists to play with in order to generate SWCNT derivatives with additional functions. In general, the covalent chemistry of carbon nanotubes can be subdivided into two fields - the modification of functional groups/defects introduced by oxidative treatments (etching) and the functionalization of pristine tubes directly via addition reactions at e.g. their sidewall (see Fig. 2.8).

The oxidative treatment of SWCNTs can be carried out either in the gas-phase by ozone/

plasma treatment or in solution with oxidizing acids such as H₂SO₄/HNO₃ or mixtures of HNO₃/H₂O₂. While the latter is widely applied in the field also for the sake of catalyst-removal or shortening of nanotubes by cutting at defect sites^[54], both meth-ods yield a whole variety of oxygen-containing defects. As shown schematically in Fig.

2.8a, these include predominantly carboxylic acids, but also hydroxyl groups, aldehy-des or ketones.^[55] These functional groups can then be further targeted and deriva-tized e.g. using thionyl chloride and alcohols or amines to form the corresponding es-ters or amides. This route and other amidation procedures were widely applied in the last two decades leading, amongst others, to SWCNT-protein^[56], -PEG^[57], -sugar^[58] or -oligonucleotide^[59,60]conjugates with possible applications in drug delivery or immunolo-gy.^[55]

Besides the derivatization of oxidized SWCNTs, there are a whole variety of other

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Figure 2.8.: Covalent carbon nanotube functionalization. a) Oxidation of SWCNTs leads to different oxygen-containing functional groups on the nanotube surface, which can subsequently be modified further by e.g. amidation reactions. b) Several examples of addition reactions leading to covalently modified SWCNTs. Whereas oxidative defects as well as reductive alkylation and [3+2] cycloadditions diminish the SWCNT’s PL (red box), a [2+1] cycloaddition by Setaro et al.^[53] as well as finely tuned reactions with diazonium salts^[39] were shown to yield nIR-fluorescent nanotubes (green box).

actions, which were exploited in the recent years for the modification of SWCNTs. A selection of those methods is presented in Fig. 2.8b. They all share the idea of having one highly reactive species, which - when in proximity to a carbon nanotube - can un-dergo an addition reaction both at its tips or at its sidewall. While the tips are typically more reactive, sidewall-defects arising e.g. from the synthesis can also lead to higher susceptibility for addition reactions on the sidewall.^[55] The variety of reactions include fluorination^[61], carbene^[62]/nitrene^[63,64] addition, Diels-Alder cycloadditions^[65], nucle-ophilic additions^[66], reductive alkylations^[67,68], free radical additions^[69,70], 1,3-dipolar cycloadditions^[71–73] or direct arylations with e.g. diazonium salts.^[74,75] Whereas flu-orinated nanotubes were shown to increase solubility in organic solvents and provide the possibility of further derivatization using e.g. Grignard-reagents or organolithium compounds^[61,76], carbene- and nitrene additions were used e.g. to attach crown-ethers or oligoethyleneglycol units.^[63] One reaction, which was exceptionally often employed, is the so-called ’Prato reaction’. It dates back to 1993, when Prato and coworkers first utilized the reactivity of azomethine ylides for the derivatization of the fullerene C₆₀.^[77]

In this reaction, azomethine ylides are formedin situvia the condensation of ana-amino acid and an aldehyde followed by a [3+2] cycloaddition to the nanotube’s sidewall or

2.2. Single-walled carbon nanotubes lizing it not only for the synthesis of water soluble SWCNTs^[78], but also for the attach-ment of peptides^[79], fluorophores^[80], cytotoxic drugs^[81], antibiotics^[82] or the multi-modal modification of nanotubes.^[83] Despite the quick adoption of this reaction, it has to be noted, that the resulting carbon nanotubes do not display their characteristic nIR photoluminescence anymore. Thus, for biological applications they lost one of their biggest advantages and consequently could only be used for imaging upon conjugation of another organic fluorophore - leaving the nanotube being no more than an attach-ment platform. To circumvent this problem, Setaro et al.^[53] made use of a different type of reaction in 2017 - a [2+1] cycloaddition with electron-poor aromatic nitrenes (see Fig. 2.8b). In particular, they used azidodichloro-triazine as a source for thein situ generated nitrene. This nitrene, in turn, can then undergo a [2+1] cycloaddition with the SWCNT’s sidewall as also observed earlier by the groups of Takagaki (2005)^[64] and Hirsch (2001)^[63] for the attachment of alkyl chains or carborane cages. In contrast to these older observations, however, Setaro et al. reported preserved nIR photolumines-cence. The authors attribute this crucial difference to the electron-poor, aromatic nature of the dichloro-triazine, which together with the high strain leads to ring-opening and rehybridization. In the next step, they used this functionalization strategy for the gen-eration of spiropyran-switchable nanotubes and conjugation of plasmonic gold nanopar-ticles leading to even further increased PL intensity.^[53] Another alternative for SWCNT functionalization is the reaction with aryl diazonium salts. This reaction was reported already by Dyke and Tour in 2004 for the modification of carbon nanotubes (and their separation from metallic SWCNTs)^[84], however, it was the laboratory of YuHuang Wang and coworkers at the University of Maryland to find the preservation and also modula-tion of the SWCNT’s PL at certain reacmodula-tion condimodula-tions. While the effect of these so-called quantum defects on the nanotube’s PL was already discussed in section 2.2.2.3, the reac-tion mechanism leading to these defects should be discussed in the following.

As stated in section 2.2.2.3, carbon nanotubes containing quantum defects could present a highly valuable tool for biomedical imaging and diagnostics. The red-shifted PL peak, which now resides at around 1130 nm, allows SWCNT excitation at theirS11-transition (approx. 990 nm for (6,5)-species) instead ofS22excitation at approx. 560 nm allowing for greatly enhanced tissue penetration and better contrast.^[46] Thus, it is highly desirable and also one of the aims of this work to use these defects not only for PL modulation, but also as an anchor for the attachment of other functional moieties such as fluorophores, peptides or proteins. The corresponding techniques for (bio)conjugation will be

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cussed in section 2.4.1. When Piao et al. observed the PL modulation in 2013^[39], they were stirring a SDS-dispersed SWCNT sample with 4-nitrobenzenediazonium tetrafluo-roborate for a prolonged time (240 h) at 25 C. Three years later, the same group pub-lished a revised procedure with drastically enhanced reaction kinetics upon SWCNT ex-citation (Fig. 2.9a).^[85] This is due to the nature of the reaction of carbon nanotubes, dispersed in water using a surfactant as e.g. SDS or SDBS, and an aryldiazonium salt 1, which proceeds viaa radical mechanism with two possibilities for the initiation step.

First, the cleavage of anin situformed diazoanhydride (2,viaa Gomberg-Bachmann re-action) can give the aryl radical3and second, the excitation of SWCNTs with (resonant, see Fig. 2.9b) light could lead to a single electron transfer (SET) from the nanotube 5

Figure 2.9.:Mechanism of defect introduction by diazonium salts. a) Diagram show-ing the difference in SWCNT-PL increase of a defect-reaction (p NO₂-Dz) with and with-out excitation. b) Absorbance spectrum of a SDS-SWCNT sample and its influence on the defect PL intensity (shown as red dots at the respective excitation wavelength). c) Free radical chain mechanism for the incorporation of defects into SWCNTs using diazonium salts. Depending on the reaction conditions, there are two possibilities for initial radical formation. In7, the radical is formed on thea-carbon, but can migrate over the extended

2.2. Single-walled carbon nanotubes onto the aryldiazonium salt yielding a aryl radical and a SWCNT-radical-cation 6 (with faster kinetics).^[85,86] Following a radical chain propagation mechanism, the formed aryl radical3 can now attack a SWCNT5resulting in a Aryl-SWCNT radical 7, which in turn can generate another aryl radical via SET or recombine with an aryl radical to form a doubly substituted SWCNT (10, see Fig. 2.9c). It is important to note, that the radi-cal in 7 is formed in 1,2-position with respect to the aryl substituent, but can migrate throughout the extended ⇡-system of the SWCNT until recombination with e.g. another aryl radical or trion formation.^[87]

If these quantum defects should now be utilized e.g. for the attachment of functional units to SWCNTs or to increase aqueous solubility, it is, however, very important to look at the number of defects introduced using diazonium chemistry. Whereas other techniques such as the Prato reaction, fluorination or the addition of nitrenes/carbenes (including the recent approach by Setaro et al.^[53]) lead to one defect for every 2-100 carbon atoms (which roughly translates to one defects per 0.1-1 nm for (6,5)-SWCNT), the carefully adjusted conditions employed by Piao et al. for the generation of quantum defects result in approximately one functional group per 10-20 nm length of (6,5)-SWCNT. This differ-ence is of crucial importance when it comes to sensing capabilities (e.g. sensor dynamic range, sensitivity) and also shielding of the hydrophobic SWCNT surfaces for aqueous solubility.