Light-induced quantum friction of carbon nanotubes in water

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Friction is a well-known phenomenon, with the first quantitative description dating back to Leonardo da Vinci (Amonton’s law). To move an object over a surface, a force proportional to the normal force (weight) is necessary. At the nanoscale, friction becomes more complex because of surface topography 1 . A fundamentally different type of friction has been theoretically proposed and coined quantum friction 2 .

It describes non-adiabatic coupling between collective modes of solvent dipoles and electronic modes of materials such as graphene, graphite and carbon nanotubes (CNTs) 2 , 4 , 5 . The friction is expected to increase when the surface response function of the substrate overlaps with the low-frequency spectrum of the solvent, including librational, intermolecular stretch and Debye modes 4 . Experimental studies have provided evidence supporting coupling beyond the Born–Oppenheimer approximation between classical water dynamics and the quantum dynamics of confined delocalized electrons. This is demonstrated by anomalies in hydrodynamic friction at water–carbon interfaces 6 , 7 and by the rapid cooling of hot electrons in graphene in water 3 . Optical pump terahertz (THz) probe experiments showed faster cooling in water compared with other solvents, which was attributed to near-field radiative heat transfer (NFRHT) between graphene surface plasmon modes and water charge fluctuations in the frequency range of the librational mode of water in the THz region 3 . THz spectroscopy can probe solute–solvent interactions 8 , including those driven by charge fluctuations 5 . Notably, these charge-fluctuation-driven interactions extend beyond the primary hydration shell 9 , influencing solvation dynamics in a broader sense. The anomalously high water friction on graphite, as well as the unique slippage behaviour observed in CNTs, has been attributed to THz plasmon modes 2 , 4 .

Semiconducting single-walled carbon nanotubes (SWCNTs) are one-dimensional nanomaterials that fluoresce in the near-infrared (NIR) tissue transparency window 10 , 11 . Their fluorescence is best described by electron–hole pairs called excitons 12 , which diffuse along the axis of the SWCNTs for around 100 nm (refs.  13 , 14 ). Excitons are affected by changes in the surrounding dielectric environment caused by bundling 15 , surfactants 16 or DNA wrapping 17 . SWCNTs themselves are hydrophobic 18 , but adsorption of surfactants, peptides, proteins 19 , 20 or π-stacking of nucleic acids 21 makes them water-soluble. Their surface chemistry can be further tuned by covalent functionalization, which introduces a low number of σ-bonds into the sp 2 hybridized carbon lattice ( sp 3 quantum defects). They act as local traps for excitons and create new photophysics 22 , 23 , 24 , 25 . These optoelectronic properties of SWCNTs are highly sensitive to their chemical environment, which makes them ideal building blocks for (bio)sensors 18 that can image chemical signalling by cells 26 , 27 , for cancer or pathogen diagnostics 28 , 29 , or to image plant stress 30 , 31 . The fluorescence changes (that is, exciton decay or energy shift) of these biosensors have been attributed to conformational changes and changes in local solvation 5 .

Here, we study whether excitons affect the diffusion of fluorescent SWCNTs in water. We use physical manipulation by changing (light) excitation, and chemical control by adding analytes or changing surface chemistry to identify how excitons affect friction and, consequently, diffusion. Based on THz spectroscopy to explore exciton–water interactions and molecular dynamics simulations, we propose a mechanism for the observed phenomena.

We conducted single-molecule fluorescence measurements to explore the diffusion behaviour of SWCNTs in water under light excitation (Fig. 1a ). For this purpose, the hydrophobic SWCNTs (mainly semiconducting (6,5)-chirality) were functionalized with single-stranded DNA ((GT) 10 ) or surfactants (deoxycholate (DOC), sodium cholate (SC) and sodium dodecyl benzene sulfonate (SDBS)) (Extended Data Figs. 1 and 2 and Supplementary Figs. 1–5 ). Moreover, we prepared SWCNTs with nitro-aryl sp 3 quantum defects, which trap and localize excitons 23 , 25 .

Fig. 1: Light-induced diffusion changes of CNTs in water. The alternative text for this image may have been generated using AI.

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a , Schematic of experimental design. SWCNTs modified with a biopolymer such as DNA are water-soluble. Upon optical excitation, excitons are created that decay by either emitting NIR photons or dissipating energy. The SWCNTs diffuse due to Brownian motion. b , Inset, FCS of (GT) 10 -SWCNTs shows an excitation power-dependent change of the fluorescence (>900 nm) autocorrelation function ( λ exc  = 480 nm). The normalized and fitted (equation ( 4 ) and Supplementary Table 1 ) autocorrelation functions indicate slower diffusion with increasing power. c , Corresponding diffusion constants of (GT) 10 -SWCNTs at…