Photoluminescence Of Single-walled Carbon Nanotubes Research Articles

Near-Infrared Photoluminescence Responses of Single-Walled Carbon Nanotubes Induced by Biomolecules Detected on a Microbead Surface.

We demonstrate near-infrared (NIR) photoluminescence (PL) microscopy of single-walled carbon nanotubes (SWNTs) attached to individual micrometer-sized bead surfaces. Monodispersed SWNTs wrapped with sodium carboxymethyl cellulose (CMC-SWNT hybrids) were attached to the microbead surfaces, and the functionalized beads were immobilized on a coverslip. It is known that NIR PL spectra of the SWNTs were quenched upon exposure to potassium permanganate (KMnO4) and recovered by the addition of papain molecules by macroscopic PL spectroscopy. In this study, the PL responses were successfully detected on a microbead surface by NIR microscopy. In particular, PL responses of (6, 5) chirality SWNTs were selectively detected by using two bandpass filters. When KMnO4 (final concentration: 1 μM) was dropped onto the coverslip surface, the PL intensity of the SWNTs decreased to approximately 20% of the initial intensity without KMnO4. The average PL intensity of each bead was estimated from the microscopy images. When a papain solution (final concentration: 5 mg/mL) was added to the same sample, the PL intensity recovered to 60-70% of the initial intensity. A similar recovery was observed with papain solutions preheated at 60 and 100 °C. The microscopic observation of the beads was performed sequentially. Although the PL responses induced by papain molecules have been reported in previous studies using macroscopic PL spectroscopy, this is the first report to demonstrate responses on a single-bead surface. Our results hold significance for investigating enzyme reactions using SWNT PL responses at the nano and micro levels.

Polarization Resolved Fluorescence Detection of Carbon Nanotube Corona Phases

Circular dichroism measures how chiral particles absorb right- and left- circularly polarized light differently to distinguish pairs of enantiomers. It can also elucidate differential photoabsorption of chiral fluorescent species via their emissions. However, this method has generally been restricted to bulk samples. Hence, these methods conceal information on the specific chiroptical response of a single particle. Furthermore, circular dichroism has been restricted in time- resolution as well as spectral range. In this work, we observed fluorescence detected circular dichroism (FDCD) of individually deposited single walled carbon nanotube (SWNT) corona. A custom-built setup compromising a linear polarizer (LP) and a quarter waveplate (QWP) was used to induce circularly polarized light (CPL). The light polarization state at the sample plane was analyzed using Stoke’s parameter. Experimental results showed that linear dichroism of the dichroic filter inside the microscope changes the polarization state of the incoming beam drastically, and thus inducing elliptically polarized light at the sample plane. A liquid crystal variable retarded (LCVR) was used to correct the polarization state of light and yield CPL at the sample plane. Singly-dispersed SWNT with different wrapping polymers were deposited onto a glass substrate. Alternating cycles of right- and left-handed circularly polarized light was delivered to excite the sample. The fluorescence intensities of each individual SWNT were tracked. The dissymmetry (g) parameters: g = 2(IL-IR)/(IL+IR) (where I L and I R are the measured fluorescence intensities with left and right circularly polarized light, respectively) were analyzed from the fluorescent signals to understand photoabsorption differences. A non-zero value of dissymmetry parameter provides information on how the corona modulates SWNT photoluminescence, an outcome not observable by bulk characterization methods. This observation can potentially enable the measurement of chiral species local interactions without a separations process.

Control of functionalized single-walled carbon nanotube photoluminescence via competition between thermal rearrangement and elimination.

We conducted the chiral separation of functionalized single-walled carbon nanotubes (SWNTs) with dibromopropane derivatives. Depending on their chirality and diameter, the thermal treatment of functionalized SWNTs leads to a shift in the emission radiation to longer wavelengths owing to rearrangement reaction in competition with elimination reaction.

Carbon Nanotube Quantum Defect Photoluminescence Modulation for Biosensors

Covalent sp3 defects on single-walled carbon nanotubes (SWCNTs), often called organic color centers (OCCs) have been explored for their photoluminescent properties, including the potential for introducing new emission bands, increasing quantum yield, and introducing new chemical sensitivities. The application of OCCs for biomedical applications requires new methods to synthesize them via covalent chemistry, to control their biological interactions/biocompatibility via supramolecular interactions with excipients, and to measure their emission in biological environments. We have found specific utilities of OCCs for the detection of chemical phenomena where SWCNT photoluminescence is normally relatively insensitive, such pH changes in the physiologic range. We developed OCCs to measure lysosomal pH in live cells and in vivo, enabling new biological discoveries. We also developed OCC-based biosensors for cancer detection. We believe that OCCs will facilitate the development of an important class of sensors for biomedical applications.

Size Selective Corona Interactions from Self-Assembled Rosette and Single-Walled Carbon Nanotubes.

Nanoparticle corona phases, especially those surrounding anisotropic particles, are central to determining their catalytic, molecular recognition, and interfacial properties. It remains a longstanding challenge to chemically synthesize and control such phases at the nanoparticle surface. In this work, the supramolecular chemistry of rosette nanotubes (RNTs), well-defined hierarchically self-assembled nanostructures formed from heteroaromatic bicyclic bases, is used to create molecularly precise and continuous corona phases on single-walled carbon nanotubes (SWCNTs). These RNT-SWCNT (RS) complexes exhibit the lowest solvent-exposed surface area (147.8± 60 m-1 ) measured to date due to its regular structure. Through Raman spectroscopy, molecular-scale control of the free volume is also observed between the two annular structures and the effects of confined water. SWCNT photoluminescence (PL) within the RNT is also modulated considerably as a function of their diameter and chirality, especially for the (11, 1) species, where a PL increase compared to other species can be attributed to their chiral angle and the RNT's inward facing electron densities. In summary, RNT chemistry is extended to the problem of chemically defining both the exterior and interior corona interfaces of an encapsulated particle, thereby opening the door to precision control of core-shell nanoparticle interfaces.

Tuning of photoluminescence intensity and Fermi level position of individual single-walled carbon nanotubes by molecule confinement

Photoluminescence of single-walled carbon nanotubes is monitored at the individual scale by molecule encapsulation into their hollow core. Depending on the electronic character (electron donor or acceptor) of the confined molecule, enhancement or quenching of the photoluminescence intensity is demonstrated. This behavior is assigned to a charge transfer, evidenced by the shift of the Raman G-band, and a correlated Fermi level shift shown by photoemission experiments. Our experimental results are supported by DFT calculations. A consistent picture of the physical interactions taking place in the hybrid systems and their effects on the optical and electronic properties is given. Our results indicate that the electron affinity or ionization potential of the encapsulated molecules and the diameter of the nanotube are relevant parameters to tune the light emission properties of the hybrid systems at the nanoscale.

(Invited) Organic Color Center Photoluminescence Modulation for Biomedical Applications

The modulation of single-walled carbon nanotube (SWCNT) photoluminescence has been under investigation for the unique potential to develop biosensors to detect many classes of analytes. Covalent sp3 defects on SWCNTs, dubbed organic color centers (OCCs), have been explored for their photoluminescent properties, including increasing quantum yield and introducing new chemical sensitivities. The application of OCCs for biomedical applications requires new methods to control SWCNT surface chemistry and the resulting biological interactions/biocompatibility via supramolecular interactions with polymers and other excipients. We have found specific utilities of OCCs for the detection of chemical phenomena where SWCNT photoluminescence is normally relatively insensitive, such as pH changes in the physiologic range of 4.5 - 8. We developed OCC-based sensors to measure pH in live cells and in vivo, as well as other emerging applications. We believe that OCCs will facilitate the development of biosensors for disease detection and biomedical research.

Development of Single-Walled Carbon Nanotube-Based Optical Sensors Via Data Analytics

The photoluminescence of single-walled carbon nanotubes (SWCNTs) offers unique advantages for the development of biological sensors, including the potential to sensitize emission to many classes of analytes. Methods to selectively transduce the local concentration of an analyte via an optical signal often involve the attachment of existing molecular recognition elements to the nanotube surface, or the development of selectivities by high-throughput screening or selection techniques. Due to the existence of several mechanisms of optical modulation of SWCNTs, as well as multiple carbon nanotube species with differing bandgaps, and the increasing number of excipients and polymers that can be used for colloidal suspension and non-covalent functionalization, data analytics can be used to facilitate selective recognition of bioanalytes to detect and process multi-parametric spectroscopic fingerprints. These strategies will be discussed for the detection of single and multiplexed analytes.

(Invited) Different Pathways of Fluorescent SWCNT Modifications with Aromatic Reactants

Modification of electronic and chemical properties of single-wall carbon nanotubes (SWCNTs) by covalent side-wall functionalization can produce a stable material with purposely-enhanced electronic and optical properties. In the report we describe a new type of reactions between simple aromatic compounds and SWCNTs, creating different fluorescent trapping states in SWCNTs. The photoreactions take place in aqueous suspensions of SWCNTs with small addition of soluble or even very-slightly soluble organic aromatic compounds excited by UV light. The reaction rates depend on the aromatic compound structure and proportional to its concentration, and in some cases, as with aniline or iodoaniline, new strong and well-resolved spectral sidebands appear within one minute. Total SWCNT photoluminescence intensity can increase, and the increase was in some cases by a factor of 5 as compare to original species. The shifted emission bands and the trapping nature of them, when the main part of the nanotube is performing as an absorbing antenna delivering excitation energy to the modified sites, may be interesting for a variety of optical and electronic applications.Most notably, in most cases of these photochemical reactions, the shifted emission spectra from modified SWCNTs become different, displaying never observed previously structure upon removal of molecular oxygen from the solution. For instance, treatment with normal dissolved oxygen gives a rise to a new emission band red-shifted for (6,5) SWCNTs by 160 meV from the pristine position, which is similar to many previously studied reactions starting with oxygen doping. Corresponding results will appear for other species as well. However, if the molecular oxygen is removed from the suspension by degassing or gas substitution, same UV treatments will lead to two different emission bands red-shifted by 140 and 270 meV, one less shifted and one more shifted, thus indicating different chemical products of the reactions. These new bands grow with different rate under UV illumination. Variance spectroscopy of such samples shows the presence of individual “multicolor” nanotubes with three distinct emission bands, one original/unmodified plus two shifted. The reaction demonstrates new possibilities for creation different trapping states on individual nanotubes, and opens new options for tuning electronic and optical properties of nanotubes for possible applications.

Organic Color Center Photoluminescence Modulation for Biomedical Applications

The modulation of single-walled carbon nanotube (SWCNT) photoluminescence has been under investigation for the unique potential to develop biosensors capable of detecting many classes of analytes. Covalent sp3 defects have introduced emissive color centers on SWCNTs, significantly enhancing photoluminescence quantum yield and chemical selectivity/sensitivity to certain analytes. The application of these organic color centers (OCCs) for biomedical applications requires new methods to both conduct covalent chemistry and to concomitantly control the surface chemistry and the resulting biological interactions/biocompatibility via their supramolecular interactions with polymers and other excipients. We will report specific utilities of OCCs for the detection of chemical phenomena where SWCNT photoluminescence is normally relatively insensitive, such as pH changes in the physiologic range of 4.5 - 8. We have developed OCCs to measure endolysosomal pH in live cells and in vivo, as well as other emerging applications. The results support the feasibility of using OCCs as biosensors for disease detection and biomedical research.

(Invited) Development of Single-Walled Carbon Nanotube-Based Optical Sensors Via Data Analytics

The photoluminescence of single-walled carbon nanotubes (SWCNTs) offers unique advantages for the development of biological sensors, including the potential to sensitize emission to many classes of analytes. Methods to selectively transduce the local concentration of an analyte via an optical signal often involves the attachment of existing molecular recognition elements to the nanotube surface, or the development of selectivities by high-throughput screening or selection techniques. Due to the existence of several mechanisms of optical modulation of SWCNTs, as well as multiple carbon nanotube species with differing bandgaps, and the increasing number of excipients and polymers that can be used for colloidal suspension and non-covalent functionalization, new methods in data analytics, sometimes dubbed ‘molecular perceptron’, can be used to facilitate selective recognition of bioanalytes to detect and process multi-parametric spectroscopic fingerprints. These methods will be discussed for the detection of single and multiplexed analytes.

(Invited) Different Fluorescent Modifications of Carbon Nanotubes with Aromatic Reactants

Covalent functionalization of single-wall carbon nanotubes (SWCNTs) can produce a stable material with electronic and optical properties enhanced purposely as compare to original species. In the present report we describe a new category of reactions between photoexcited aromatic compounds and SWCNTs, creating different fluorescent trapping states in SWCNTs. The reactions take place in aqueous suspensions of SWCNTs with small addition of soluble or even very-slightly soluble organic aromatic compounds excited by UV light. The reaction rate depends on the aromatic compound structure and proportional to its concentration, and in some cases, as with aniline or iodoaniline, these strong spectral sidebands appear within one minute. Total SWCNT photoluminescence intensity can be increased by a factor as large as 5. Shifted emission bands and the trapping nature of them may be interesting for a variety of optical and electronic applications.Most notably, in most cases of these photochemical reactions, the emission spectra of altered SWCNTs become different, never observed previously, upon removal of molecular oxygen from the solution, thus indicating different chemical products of the reactions. For instance, for (6,5) SWCNTs, treatment in oxygen presence gives a new emission band red-shifted by 160 meV from the pristine position, which is similar to many previously studied reactions starting with oxygen doping. However, same UV treatments after dissolved molecular oxygen removal will lead to two new emission bands red-shifted by 140 and 270 meV, growing with different rate under UV illumination. Variance spectroscopy shows the presence of individual “multicolor” nanotubes with three distinct emission bands, original plus two shifted. The reaction demonstrates new possibilities for creation different trapping states on individual nanotubes, and opens new options for tuning electronic and optical properties of nanotubes for possible applications.

Photoexcited Aromatic Reactants Give Multicolor Carbon Nanotube Fluorescence from Quantum Defects.

Covalent functionalization of single-wall carbon nanotubes (SWCNTs) can be valuable for modifying their electronic properties and creating fluorescent quantum defects. We report here a previously unreported category of such reactions involving interactions of photoexcited aromatic compounds with SWCNT sidewalls. When aqueous suspensions of SWCNTs are exposed to organic aromatic compounds and then irradiated by UV light, fluorescent defects are formed in the nanotubes at rates that depend on the aromatic ring substituents. In reactions with aniline or iodoaniline, strong spectral sidebands appear within 1 min. Total SWCNT photoluminescence can be enhanced by a factor as large as ∼5. Notably, emission spectra of reacted SWCNTs depend on the presence or absence of dissolved oxygen during the reaction. For (6,5) SWCNTs, treatment when oxygen is present gives an additional emission band red-shifted by 160 meV from the pristine position, whereas treatment without oxygen leads to two additional emission bands red-shifted by 140 and 270 meV. Variance spectroscopy shows the presence of individual "multicolor" nanotubes with three distinct emission bands (pristine plus two shifted). The facile generation of dual fluorescent quantum defects in SWCNTs provides emission closer to standard telecom wavelengths, advancing the prospects for applications as single-photon sources in quantum information processing.

Modulating Chirality-Selective Photoluminescence of Single-Walled Carbon Nanotubes by Ionic Liquids.

The chirality-selective near-infrared emission of surfactant-stabilized single-wall carbon nanotubes could be controlled by simply varying the anion of the commonly used 1-butyl-3-methylimidazolium ionic liquids. This result advances the notion of the designer solvent ability of ionic liquids and provides opportunities for modulating the properties of nanomaterials.

(Invited) Carbon Nanotube Photoluminescence Spectroscopy for Applications in Cancer Research

The detection of biomarkers and drugs in live cells and animals would allow for more effective studies of the dynamics of cancer and treatment. Toward these ends, single-walled carbon nanotube photoluminescence is suitable for measurements in live cells and in vivo, and nanotube near-infrared emission exhibits sensitivity to the local environment via solvatochromic responses. We developed new optical instrumentation to image carbon nanotubes and to measure spectral changes of carbon nanotube photoluminescence in live cells and animals. Concomitantly, we developed new methods to exacerbate the nanotube emission response to produce large spectral shifts (> 14 nm) upon the binding of cancer biomarkers and chemotherapeutic drugs. The ability to improve measurements and control of carbon nanotube solvatochromism may improve sensitivity and selectivity of nanosensors and potentially expand the application of nanotube-based optical measurements to new classes of bioanalytes.

(Invited) Developments in Modulating Carbon Nanotube Photoluminescence

The photoluminescence of single-walled carbon nanotubes may facilitate quantification of analytes within complex environments. To design quantitative sensors using carbon nanotubes, greater understanding of the optical modulation of their photoluminescence is necessary. On exploring the modulation of nanotubes, we found new methods to enable unprecedented control over the shifting of emission wavelength towards both red and blue wavelengths, facilitated by adsorbed species including biomacromolecules and polymers that allow for switchable modification of surface coverage and charge on the nanotube surface.

(Invited) Effect of Physisorbed Oxygen on the Photoluminescence of Single-Walled Carbon Nanotubes Suspended By Single-Stranded DNAs

For more than a decade, DNA and oligonucleotides of single-strand DNA (ssDNA) have studied as agents for dispersing single-walled carbon nanotubes (SWCNT), with the goals of potential biomedical applications and the selective sorting of specific (n,m) structures from as-produced inhomogeneous mixtures. We have recently discovered that SWCNTs suspended by ssDNA oligos in water show rather strong quenching from dissolved molecular oxygen. In many cases dissolved oxygen in equilibrium with 1 atm partial pressure of O2 decreases the SWCNT fluorescence intensity by a factor of two, and for some systems the quenching factor is two in air-saturated solutions and reaches three under 1 atm of pure O2. We find that the quenching depends on (n,m) species and ssDNA sequence, in addition to some other experimental parameters. The effect varies smoothly with dissolved oxygen concentration and is fully reversible on purging with other gases such as argon or nitrogen. The quenching effect appears and disappears on a sub-second time scale. An important finding is that the extent of oxygen quenching correlates with the apparent affinity between the DNA oligo and the SWCNT species. Combinations thought to give strong coating show weak quenching, possibly because of reduced exposure of the nanotube surface to the surrounding solution. Thus, this effect can provide a simple way to identify special affinities just by measuring fluorescence changes when dissolved oxygen is displaced by inert gases. To help understand the mechanism of the quenching effect and its potential value in other applications, we have studied a variety ssDNA oligos, including those containing only one nucleotide. We have also studied the effect with other dispersing agents and surfactants, quantified the dependence of quenching on oxygen concentration, analyzed fluorescence spectral shifts caused by dissolved oxygen, and explored the quenching effect in solid nanotube films and in solutions modified by addition of polymers and other chemicals. We will present the results of those experiments and our proposed mechanism for the effect.

Defect-Induced Near-Infrared Photoluminescence of Single-Walled Carbon Nanotubes Treated with Polyunsaturated Fatty Acids.

Single-walled carbon nanotubes (SWCNTs) have been incorporated in many emerging applications in the biomedical field including chemical sensing, biological imaging, drug delivery, and photothermal therapy. To overcome inherent hydrophobicity and improve their biocompatibility, pristine SWCNTs are often coated with surfactants, polymers, DNA, proteins, or lipids. In this paper, we report the effect of polyunsaturated fatty acids (PUFAs) on SWCNT photoluminescence. A decrease in the SWCNT bandgap emission (E11) and a new red-shifted emission (E11-) were observed in the presence of PUFAs. We attribute the change in SWCNT photoluminescence to the formation of oxygen-containing defects by lipid hydroperoxides through photo-oxidation. The observed changes in near-infrared emission of SWCNTs are important for understanding the interaction between SWCNTs and lipid biocorona. Our results also indicate that photoexcited SWCNTs can catalyze lipid peroxidation similarly to lipoxygenases.

Single-walled carbon nanotubes as optical probes for bio-sensing and imaging.

The unique physical properties of single-walled carbon nanotubes (SWCNTs) have been exploited in novel applications in various fields including electronics and life sciences. Their photoluminescence in the near-infrared (NIR) range, where optical interference from biological tissues is minimum, has rendered them particularly attractive as optical probes in biological environments. Herein we review the use of the SWCNT NIR emission in bio-sensing and imaging. To interface the insoluble carbon nanotubes with an aqueous biological environment, biomaterials and organic polymers have been widely used for non-covalently functionalizing SWCNTs. Such functionalization minimizes the toxicity of carbon nanotubes in biological and physiological environments, while maintaining their optical properties. SWCNTs have been demonstrated as both in vitro and in vivo optical sensors, targeting biologically important molecules, such as neurotransmitters and cell signaling molecules. For optical imaging, functionalized SWCNTs were used as NIR contrast agents for probing cellular processes and imaging plants and small animals. We also discuss emerging SWCNT-based super-resolution schemes. We conclude that SWCNTs are promising optical materials for basic life science research, biomedical diagnostics, and therapeutics.

Two-color spectroscopy of UV excited ssDNA complex with a single-wall nanotube photoluminescence probe: Fast relaxation by nucleobase autoionization mechanism

DNA autoionization is a fundamental process wherein UV-photoexcited nucleobases dissipate energy by charge transfer to the environment without undergoing chemical damage. Here, single-wall carbon nanotubes (SWNT) are explored as a photoluminescent reporter for studying the mechanism and rates of DNA autoionization. Two-color photoluminescence spectroscopy allows separate photoexcitation of the DNA and the SWNTs in the UV and visible range, respectively. A strong SWNT photoluminescence quenching is observed when the UV pump is resonant with the DNA absorption, consistent with charge transfer from the excited states of the DNA to the SWNT. Semiempirical calculations of the DNA-SWNT electronic structure, combined with a Green's function theory for charge transfer, show a 20 fs autoionization rate, dominated by the hole transfer. Rate-equation analysis of the spectroscopy data confirms that the quenching rate is limited by the thermalization of the free charge carriers transferred to the nanotube reservoir. The developed approach has a great potential for monitoring DNA excitation, autoionization, and chemical damage both {\it in vivo} and {\it in vitro}.