Application as optical sensors

As described in the previous sections, carbon nanotubes are special from many points of view. Their 1D tubular nature renders every (carbon) atom building up the tube a surface atom. This has one immediate consequence: The nanotube’s optoelectronic properties are highly sensitive to changes in the SWCNT’s surrounding. This can be observed im-mediately when looking at the absorption maxima of SWCNTs wrapped with different surfactants varying over approx. 20 nm.^[88] This change depending on the nature, con-formation and surface-coverage of the encapsulating molecule can be broken down to changes in the dielectric environment in general.^[34] Here, an increasing dielectric con-stant is causing a red-shifted absorbance/emission as well as a decrease of exciton oscil-lator strength.^[34] Both the redshift and the decrease of exciton oscillator strength can be attributed to dielectric screening of excitons by solvent molecules with the effect of en-hanced non-radiative recombination or exciton dissociation.^[89,90] However, this effect is not uniform in nature as SWCNTs were also shown to display different PLQY in solvents

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2. Introduction

of similar polarity. Larsen et al. found, that the additional important factor is solvent electrophilicity, which could lead to a shift of electron density from the tubular surface by electrophilic solvents. This, in turn, could lead to more non-radiative recombination sites and consequently reduced PLQY.^[89]

This dependence on the dielectric properties as well as on the electrophilicity of the surrounding environment holds true not only for (organic) solvents, but also for sur-factants, polymers or even solute molecules in general.^[34] While (bio)polymers or sur-factants, as described in section 2.2.3.1, can form micelles around SWCNTs or directly adsorb on their hydrophobic surface and thus directly impact the dielectric environment, solute molecules can intermittently interfere with this coating and in that way cause dielectric perturbations. Using near-infrared fluorescence spectroscopy, these dielectric perturbations can be probed either in terms of changes in PL intensity or wavelength shifts. Mechanistically, there are several approaches discussed in literature including sol-vatochromism, charge-transfer or doping/redox-reactions.^[11] While redox reactions or doping results in increased/decreased non-radiative exciton recombination sites (! de-creased/increased PLQY), charge-transfer e.g. from the SWCNT’s valence band to the analytes’ LUMO leads to altered population of ground/excited state and thus changed exciton relaxation kinetics (!decreased/increased PLQY).^[11]

Whereas the PL of SWCNTs is highly sensitive to its environment and is able to report on changes via PL intensity- or wavelength modulation (detection unit), a sensor addi-tionally requires both a recognition unit (e.g. antibody or aptamer) as well as a signal transduction unit (e.g. molecular linker, enzyme) for selective binding and detection of a target analyte. With respect to SWCNT-based sensors, they can roughly be sorted into two categories. First, those relying on known recognition motifs or known inter-actions and second, sensors found based on a screening approach making use of the unique structural confinement of biopolymers on the hydrophobic SWCNT surface lead-ing to new recognition motifs.^[91] A few examples for SWCNT-based optical sensors de-veloped during the last 15 years are shown in Fig. 2.10. SWCNT-based optical sensors cover a wide range of analytes from reactive oxygen/nitrogen species (ROS/RNS) such as NO^[98], OH radicals^[99] or H₂O₂^[100] over small molecules as riboflavin^[48], glucose^[92]

or dopamine^[5] to larger biomolecules (e.g. DNA^[101,102], glycan-profiling^[103], single nu-cleotide polymorphisms^[104]) or even whole proteins (e.g. fibrinogen^[50], HE4^[96]). By comparing the sensors targeting these very different molecules, the differences in sensor readout are very much apparent comprising PL wavelength shifts or PL intensity

modula-2.2. Single-walled carbon nanotubes

Figure 2.10.: SWCNT-based optical sensors. Overview on different mode of actions of several sensors and their respective sensor readout (top right boxes). a) Examples of SWCNT-based optical sensors utilizing known recognition motifs or interactions for the detection of glucose^[92], the neurotransmitter serotonin^[93], H₂O₂ released during plant stress^[94], several proteins^[95] or the ovarian cancer biomarker HE4.^[96] b) SWCNT-based optical sensors based on structurally constrained (bio)polymers pinned to the SWCNT-surface detecting dopamine^[97], fibrinogen^[50] or riboflavin.^[48]Figures adapted and mod-ified with permission from Yum et al.^[92], Dinarvand et al.^[93], Wu et al.^[94], Ahn et al.^[95], Williams et al.^[96], Kruss et al.^[97], Bisker et al.^[50] and Zhang et al.^[48]

tion caused by the phenomena discussed above. The sensors developed for ROS/RNS re-spond most likely to adsorption of these redox-active molecules on the SWCNT’s surface and thus show quenched PL, which was utilized e.g. for the spatiotemporal monitoring of NO-production inside macrophage cells.^[105] The same readout mode (PL quenching) is observed for many protein sensors as e.g. those shown in Fig. 2.10a/b for fibrinogen or the p16-CDK4/Jun-Fos pairs. While for the latter case the PL quenching is caused by Ni²⁺-induced proximity quenching, which is enhanced upon binding of the target protein (after a conformational change of the "bait" protein), Bisker et al. attribute the selective PL quenching to a combined effect of molecular recognition by the phospholipid-PEG

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2. Introduction

corona-phase and the unique 3D-structure of fibrinogen.^[50] In contrast, glucose or the neurotransmitter dopamine are detected with an increase of PL caused by a conforma-tional change of the wrapping (GT)₁₅ oligonucleotide (dopamine)^[5]or boronic ester for-mation (glucose).^[92] Besides PL quenching or enhancement, a wavelength shift can also be a tool for analyte detection. Among others, this was utilized for the optical sensing of riboflavin or the ovarian cancer biomarker HE4 (human epididymis protein 4), where Williams et al.^[96] made use of a anti-HE4 antibody as a detection unit. The wavelength-shift was attributed to solvatochromism induced by polymer dielectric changes^[48] or removal of water upon binding of the HE4-protein resulting in a reduction of the local dielectric constant as discussed above.^[96]

While Fig. 2.10 highlights in vitro applications of SWCNT-based sensors, the beneficial properties of nIR-PL such as enhanced tissue penetration depth or the absence of bleach-ing/blinking led to quick adoption of a variety of the described sensors in in cellulo or evenin vivoapplications. The dopamine sensor developed by Kruss et al.^[5] was later ap-plied for the monitoring of dopamine secretion from stimulated PC12 cells allowing for a high spatiotemporal resolution surpassing existing electrochemical techniques.^[97] Fur-thermore, several other nanotube-based sensors were even applied in live brain slices^[46]

or mice^[96]. It is now part of this work to further enhance SWCNT-based optical sensors and to find other chemistries allowing for a more general approach of sensor generation.