Aggregation Control of α-Sexithiophene via Isothermal Encapsulation Inside Single-Walled Carbon Nanotubes

Performance of Particle Swarm Optimization in Predicting the Orientation of π-Conjugated Molecules Inside Carbon Nanotubes Compared with Density Functional Theory Calculations

Particle swarm optimization (PSO) was employed to obtain the global minimum of host-guest structures consisting of a triiodobenzene molecule (BzI3) inside an armchair (m,m) nanotube (BzI3@(m,m)), whose host-guest interactions are approximated by Lennard-Jones (LJ) potentials. The host-guest structures obtained using the PSO-LJ method were then compared with those obtained through dispersion-corrected density functional theory (DFT) calculations to evaluate the performance of the PSO-LJ approach in predicting the guest orientation inside a tube. When the inner space of the host tube is limited for guest encapsulation, the PSO-LJ method can reproduce the DFT results of BzI3@(m,m) in terms of the guest orientation. Conversely, in nanotubes with a sufficiently large space to allow a guest to freely move, corresponding to weak tube confinement, the PSO-LJ method yields guest orientations that are different from those obtained through DFT calculations; however, both methods obtain energetically close guest orientations. Accordingly, PSO-LJ method-assisted DFT calculations can quickly provide energetically stable guest orientations in BzI3@(m,m) in weak tube confinement, which can be ignored in DFT calculations, where a single initial geometry is typically used.

Photoinduced Dynamics of a Single-Walled Carbon Nanotube with a sp3 Defect: The Importance of Excitonic Effects

Herein, we employed linear-response time-dependent functional theory nonadiabatic dynamic simulations to explore the photoinduced exciton dynamics of a chiral single-walled carbon nanotube CNT(6,5) covalently doped with a 4-nitrobenzyl group (CNT65-NO2). The results indicate that the introduction of a sp3 defect leads to the splitting of the degenerate VBM/VBM-1 and CBM/CBM+1 states. Both the VBM upshift and the CBM downshift are responsible for the experimentally observed redshifted E11* trapping state. The simulations reveal that the photoinduced exciton relaxation dynamics completes within 500 fs, which is consistent with the experimental work. On the other hand, we also conducted the nonadiabatic carrier (electron and hole) dynamic simulations, which completely ignore the excitonic effects. The comparison demonstrates that excitonic effects are indispensable. Deep analyses show that such effects induce several dark states, which play an important role in regulating the photoinduced dynamics of CNT65-NO2. The present work demonstrates the importance of including excitonic effects in simulating photoinduced processes of carbon nanotubes. In addition, it not only rationalizes previous experiments but also provides valuable insights that will help in the future rational design of novel covalently doped carbon nanotubes with superior photoluminescent properties.

Duplex Phenotype Detection and Targeting of Breast Cancer Cells Using Nanotube Nanoprobes and Raman Imaging

We designed, synthesized, and characterized a Raman nanoprobe made of dye-sensitized single-walled carbon nanotubes (SWCNTs) that can selectively target biomarkers of breast cancer cells. The nanoprobe is composed of Raman-active dyes encapsulated inside a SWCNT, whose surface is covalently grafted with poly(ethylene glycol) (PEG) at a density of ∼0.7% per carbon. Using α-sexithiophene- and β-carotene-derived nanoprobes covalently bound to an antibody, either anti-E-cadherin (E-cad) or anti-keratin-19 (KRT19), we prepared two distinct nanoprobes that specifically recognize biomarkers on breast cancer cells. Immunogold experiments and transmission electron microscopy (TEM) images are first used to guide the synthesis protocol for higher PEG-antibody attachment and biomolecule loading capacity. The duplex of nanoprobes was then applied to target E-cad and KRT19 biomarkers in T47D and MDA-MB-231 breast cancer cell lines. Hyperspectral imaging of specific Raman bands allows for simultaneous detection of this nanoprobe duplex on target cells without the need for additional filters or subsequent incubation steps. Our results confirm the high reproducibility of the nanoprobe design for duplex detection and highlight the potential of Raman imaging for advanced biomedical applications in oncology.

Fluorescence anisotropy using highly polarized emitting dyes confined inside BNNTs

Polarized fluorescence emission of nanoscale emitters has been extensively studied for applications such as bioimaging, displays, and optical communication. Extending the polarization properties in large assemblies of compact emitters is, however, challenging because of self-aggregation processes, which can induce depolarization effects, quenching, and cancellations of molecular dipoles. Here we use α-sexithiophene (6T) molecules confined inside boron nitride nanotubes (6T@BNNTs) to induce fluorescence anisotropy in a transparent host. The experiments first indicate that individual 6T@BNNTs exhibit a high polarization extinction ratio, up to 700, at room temperature. Using aberration-corrected HRTEM, we show that the fluorescence anisotropy is consistent with a general alignment of encapsulated 6T molecules along the nanotube axis. The molecular alignment is weakly influenced by the nanotube diameter, a phenomenon ascribed to stronger molecule-to-sidewall interactions compared to intermolecular interactions. By stretching a flexible thin film made of transparent polymers mixed with 6T@BNNTs, we induce a macroscopic fluorescence anisotropy within the film. This work demonstrates that the dyes@BNNT system can be used as an easy-to-handle platform to induce fluorescence anisotropy in photonic materials.

Diameter-dependent single- and double-file stacking of squaraine dye molecules inside chirality-sorted single-wall carbon nanotubes

The filling of single-wall carbon nanotubes (SWCNTs) with dye molecules has become a novel path to add new functionalities through the mutual interaction of confined dyes and host SWCNTs. In particular cases, the encapsulated dye molecules form strongly interacting molecular arrays and these result in severely altered optical properties of the dye molecules. Here, we present the encapsulation of a squaraine dye inside semiconducting chirality-sorted SWCNTs with diameters ranging from ∼1.15 nm, in which the dye molecules can only be encapsulated in a single-file molecular arrangement, up to ∼1.5 nm, in which two or three molecular files can fit side-by-side. Through the chirality-selective observation of energy transfer from the dye molecules to the surrounding SWCNTs, we find that the absorption wavelength of the dye follows a peculiar SWCNT diameter dependence, originating from the specific stacking of the dye inside the host SWCNTs. Corroborated by a theoretical model, we find that for each SWCNT diameter, the dye molecules adopt a close packing geometry, resulting in tunable optical properties of the hybrid when selecting a specific SWCNT chirality.

Nanotube-Based 1D Heterostructures Coupled by van der Waals Forces

1D van der Waals heterostructures based on carbon nanotube templates are raising a lot of excitement due to the possibility of creating new optical and electronic properties, by either confining molecules inside their hollow core or by adding layers on the outside of the nanotube. In contrast to their 2D analogs, where the number of layers, atomic type and relative orientation of the constituting layers are the main parameters defining physical properties, 1D heterostructures provide an additional degree of freedom, i.e., their specific diameter and chiral structure, for engineering their characteristics. The current state-of-the-art in synthesizing 1D heterostructures are discussed here, in particular focusing on their resulting optical properties, and details the vast parameter space that can be used to design heterostructures with custom-built properties that can be integrated into a large variety of applications. First, the effects of van der Waals coupling on the properties of the simplest and best-studied 1D heterostructure, namely a double-walled carbon nanotube, are described, and then heterostructures built from the inside and the outside are considered, which all use a nanotube as a template, and, finally, an outlook is provided for the future of this research field.

Optical Property Tuning of Single-Wall Carbon Nanotubes by Endohedral Encapsulation of a Wide Variety of Dielectric Molecules

Specific and tunable modification to the optical properties of single-wall carbon nanotubes (SWCNTs) is demonstrated through direct encapsulation into the nanotube interior of guest molecules with widely varying static dielectric constants. Filled through simple ingestion of the guest molecule, each SWCNT population is demonstrated to display a robust modification to absorbance, fluorescence, and Raman spectra. Over 30 distinct compounds, covering static dielectric constants from 1.8 to 109, are inserted in large diameter SWCNTs (d = 1.104-1.524 nm) and more than 10 compounds in small diameter SWCNTs (d = 0.747-1.153 nm), demonstrating that the general effect of filler dielectric on the nanotube optical properties is a monotonic energy reduction (red-shifting) of the optical transitions with increased magnitude of the dielectric constant. Systematic fitting of the two-dimensional fluorescence-excitation and Raman spectra additionally enables determination of the critical filling diameter for each molecule and distinguishing of overall trends from specific guest-host interactions. Comparisons to predictions from existing theory are presented, and specific guest molecule/SWCNT chirality combinations that disobey the general trend and theory are identified. A general increase of the fluorescence intensity and line narrowing is observed for low dielectric constants, with long linear alkane filled SWCNTs exhibiting emission intensities approaching those of empty SWCNTs. These results demonstrate an exploitable modulation in the optical properties of SWCNTs and provide a foundation for examining higher-order effects, such as due to nonbulk-like molecule stacking, in host-guest interactions in well-controlled nanopore size materials.

Confinement of Dyes inside Boron Nitride Nanotubes: Photostable and Shifted Fluorescence down to the Near Infrared

Fluorescence is ubiquitous in life science and used in many fields of research ranging from ecology to medicine. Among the most common fluorogenic compounds, dyes are being exploited in bioimaging for their outstanding optical properties from UV down to the near IR (NIR). However, dye molecules are often toxic to living organisms and photodegradable, which limits the time window for in vivo experiments. Here, it is demonstrated that organic dye molecules are passivated and photostable when they are encapsulated inside a boron nitride nanotube (dyes@BNNT). The results show that the BNNTs drive an aggregation of the encapsulated dyes, which induces a redshifted fluorescence from visible to NIR-II. The fluorescence remains strong and stable, exempt of bleaching and blinking, over a time scale longer than that of free dyes by more than 10⁴ . This passivation also reduces the toxicity of the dyes and induces exceptional chemical robustness, even in harsh conditions. These properties are highlighted in bioimaging where the dyes@BNNT nanohybrids are used as fluorescent nanoprobes for in vivo monitoring of Daphnia Pulex microorganisms and for diffusion tracking on human hepatoblastoma cells with two-photon imaging.

Reversible Charge Transfer with Single-Walled Carbon Nanotubes Upon Harvesting the Low Energy Part of the Solar Spectrum

Here, the ability of a novel near-infrared dye to noncovalently self-assemble onto the surface of single-walled carbon nanotubes (SWCNTs) driven by charge-transfer interactions is demonstrated. Steady-state, Raman, and transient absorption spectroscopies corroborate the electron donating character of the near-infrared dye when combined with SWCNTs, in the form of fluorescence quenching of the excited state of the dye, n-doping of SWCNTs, and reversible charge transfer, respectively. Formation of the one-electron oxidized dye as a result of interactions with SWCNTs is supported by spectroelectrochemical measurements. The ultrafast electronic process in the near-infrared dye, once immobilized onto SWCNTs, starts with the formation of excited states, which decay to the ground state via the intermediate population of a fully charge-separated state, with characteristic time constants for the charge separation of 1.5 ps and charge recombination of 25 ps, as derived from the multiwavelength global analysis. Of great relevance is the fact that charge-transfer occurs from the hot excited state of the near-infrared dye to SWCNTs.

Present and Future of Surface-Enhanced Raman Scattering

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

Resonant, Plasmonic Raman Enhancement of α-6T Molecules Encapsulated in Carbon Nanotubes

Surface-enhanced Raman scattering (SERS) and resonant Raman scattering are widely used techniques to enhance the Raman intensity of molecules and nanomaterials by several orders of magnitude. In SERS, typically, molecules are investigated and their intrinsic resonance is often ignored while discussing the plasmonic enhancement. Here, we study α-sexithiophenes encapsulated in carbon nanotubes placed in the center of a nanodimer. By dielectrophoretic deposition, we place the nanotubes precisely in the center of a plasmonic gold nanodimer and observe SERS enhancement from individual tube bundles. The encapsulated molecules are not subjected to chemical enhancement because of the protective character of the nanotube. Polarization-dependent Raman measurements confirm the alignment of the molecules within the carbon nanotubes (CNTs) and reveal the influence of the plasmonic near field on the molecule's Raman intensity. We investigate the encapsulated molecules in small CNT bundles with and without plasmonic enhancement and determine the molecular and plasmonic resonance by tuning the excitation wavelength. We observe a strong red shift of the maximum Raman intensity under plasmonic enhancement toward the plasmon resonance.

Diameter-Dependent Optical Absorption and Excitation Energy Transfer from Encapsulated Dye Molecules toward Single-Walled Carbon Nanotubes

The hollow cores and well-defined diameters of single-walled carbon nanotubes (SWCNTs) allow for creation of one-dimensional hybrid structures by encapsulation of various molecules. Absorption and near-infrared photoluminescence-excitation (PLE) spectroscopy reveal that the absorption spectrum of encapsulated 1,3-bis[4-(dimethylamino)phenyl]-squaraine dye molecules inside SWCNTs is modulated by the SWCNT diameter, as observed through excitation energy transfer (EET) from the encapsulated molecules to the SWCNTs, implying a strongly diameter-dependent stacking of the molecules inside the SWCNTs. Transient absorption spectroscopy, simultaneously probing the encapsulated dyes and the host SWCNTs, demonstrates this EET, which can be used as a route to diameter-dependent photosensitization, to be fast (sub-picosecond). A wide series of SWCNT samples is systematically characterized by absorption, PLE, and resonant Raman scattering (RRS), also identifying the critical diameter for squaraine filling. In addition, we find that SWCNT filling does not limit the selectivity of subsequent separation protocols (including polyfluorene polymers for isolating only semiconducting SWCNTs and aqueous two-phase separation for enrichment of specific SWCNT chiralities). The design of these functional hybrid systems, with tunable dye absorption, fast and efficient EET, and the ability to remove all metallic SWCNTs by subsequent separation, demonstrates potential for implementation in photoconversion devices.

Band gap modification and photoluminescence enhancement of graphene nanoribbon filled single-walled carbon nanotubes

Molecule encapsulation inside the single-walled carbon nanotube (SWCNT) core has been demonstrated to be a successful route for the modification of nanotube properties. SWCNT diameter-dependent filling results in band gap modification together with the enhancement of photoluminescence quantum yield. However, the interaction between the inner structure and the outer shell is complex. It depends on the orientation of the molecules inside, the geometry of the host nanotube and on several other mechanisms determining the resulting properties of the hybrid nanosystem. In this work we study the influence of encapsulated graphene nanoribbons on the optical properties of the host single-walled carbon nanotubes. The interplay of strain and dielectric screening caused by the internal environment of the nanotube affects its band gap. The photoluminescence of the filled nanotubes becomes enhanced when the graphene nanoribbons are polymerized inside the SWCNTs at low temperatures. We show a gradual photoluminescence quenching together with a selective signal enhancement for exact nanotube geometries, specifically (14,6) and (13,8) species. A precise adjustment of the optical properties and an enhancement of the photoluminescence quantum yield upon filling for nanotubes with specific diameters were assigned to optimal organization of the inner structures.

Davydov Splitting and Self-Organization in a Porphyrin Layer Noncovalently Attached to Single Wall Carbon Nanotubes

We study the ability of porphyrin molecules to cooperate upon adsorption on the sp² curved surface of carbon nanotube. We discuss the role of the phenyl substituents in the cooperativity of the functionalization reaction. Moreover, a specific spatial organization of the molecules around the nanotube is unveiled through polarization sensitive experiments. Furthermore, we observe an increase of the energy splitting of the porphyrin main transition upon the adsorption on the nanotube. This effect, interpreted as a Davydov splitting, is analyzed quantitatively using a dipole-dipole coupling model. This study demonstrates the ability of porphyrin molecules to create an organized self-assembled layer at the surface of the nanotubes where molecules are electronically coupled together.

Plasmonic enhancement of SERS measured on molecules in carbon nanotubes

We isolated the plasmonic contribution to surface-enhanced Raman scattering (SERS) and found it to be much stronger than expected. Organic dyes encapsulated in single-walled carbon nanotubes are ideal probes for quantifying plasmonic enhancement in a Raman experiment. The molecules are chemically protected through the nanotube wall and spatially isolated from the metal, which prevents enhancement by chemical means and through surface roughness. The tubes carry molecules into SERS hotspots, thereby defining molecular position and making it accessible for structural characterization with atomic-force and electron microscopy. We measured a SERS enhancement factor of 10⁶ on α-sexithiophene (6T) molecules in the gap of a plasmonic nanodimer. This is two orders of magnitude stronger than predicted by the electromagnetic enhancement theory (10⁴). We discuss various phenomena that may explain the discrepancy (including hybridization, static and dynamic charge transfer, surface roughness, uncertainties in molecular position and orientation), but found all of them lacking in enhancement for our probe system. We suggest that plasmonic enhancement in SERS is, in fact, much stronger than currently anticipated. We discuss novel approaches for treating SERS quantum mechanically that appear promising for predicting correct enhancement factors. Our findings have important consequences on the understanding of SERS as well as for designing and optimizing plasmonic substrates.

Importance of the alignment of polar π conjugated molecules inside carbon nanotubes in determining second-order non-linear optical properties

Dispersion-corrected DFT calculations found energetically preferred alignments of certain p,p′-dimethylaminonitrostilbene (DANS) molecules inside an carbon nanotube, and their importance in determining second-order non-linear optical properties.