A Spin-Coated Hydrogel Platform Enables Accurate Investigation of Immobilized Individual Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes as near-infrared fluorescent probes for bio-inspired supramolecular self-assembled hydrogels

Hydrogels derived from fluorenylmethoxycarbonyl (Fmoc)-conjugated amino acids and peptides demonstrate remarkable potential in biomedical applications, including drug delivery, tissue regeneration, and tissue engineering. These hydrogels can be injectable, offering a minimally invasive approach to hydrogel implantation. Given their potential for prolonged application, there is a need for non-destructive evaluation of their properties over extended periods. Thus, we introduce a hydrogel characterization platform employing single-walled carbon nanotubes (SWCNTs) as near-infrared (NIR) fluorescent probes. Our approach involves generating supramolecular self-assembling hydrogels from aromatic Fmoc-amino acids. Integrating SWCNTs into the hydrogels maintains their structural and mechanical properties, establishing SWCNTs as optical probes for hydrogels. We demonstrate that the SWCNT NIR-fluorescence changes during the gelation process correlate to rheological changes within the hydrogels. Additionally, single particle tracking of SWCNTs incorporated in the hydrogels provides insights into differences in hydrogel morphologies. Furthermore, the disassembly process of the hydrogels can be monitored through the SWCNT fluorescence modulation. The unique attribute of SWCNTs as non-photobleaching fluorescent sensors, emitting at the biologically transparent window, offers a non-destructive method for studying hydrogel dynamics over extended periods. This platform could be applied to a wide range of self-assembling hydrogels to advance our understanding and applications of supramolecular assembly technologies.

Rationally Designed Functionalization of Single-Walled Carbon Nanotubes for Real-Time Monitoring of Cholinesterase Activity and Inhibition in Plasma

Enzymes play a pivotal role in regulating numerous bodily functions. Thus, there is a growing need for developing sensors enabling real-time monitoring of enzymatic activity and inhibition. The activity and inhibition of cholinesterase (CHE) enzymes in blood plasma are fluorometrically monitored using near-infrared (NIR) fluorescent single-walled carbon nanotubes (SWCNTs) as probes, strategically functionalized with myristoylcholine (MC)- the substrate of CHE. A significant decrease in the fluorescence intensity of MC-suspended SWCNTs upon interaction with CHE is observed, attributed to the hydrolysis of the MC corona phase of the SWCNTs by CHE. Complementary measurements for quantifying choline, the product of MC hydrolysis, reveal a correlation between the fluorescence intensity decrease and the amount of released choline, rendering the SWCNTs optical sensors with real-time feedback in the NIR biologically transparent spectral range. Moreover, when synthetic and naturally abundant inhibitors inhibit the CHE enzymes present in blood plasma, no significant modulations of the MC-SWCNT fluorescence are observed, allowing effective detection of CHE inhibition. The rationally designed SWCNT sensors platform for monitoring of enzymatic activity and inhibition in clinically relevant samples is envisioned to not only advance the field of clinical diagnostics but also deepen further understanding of enzyme-related processes in complex biological fluids.

Enhancing Intracellular Optical Performance and Stability of Engineered Nanomaterials via Aqueous Two-Phase Purification

Supramolecular hybrids of DNA and single-walled carbon nanotubes (SWCNTs) have been introduced in numerous biosensing applications due to their unique optical properties. Recent aqueous two-phase (ATP) purification methods for SWCNTs have gained popularity by introducing specificity and homogeneity into the sensor design process. Using murine macrophages probed by near-infrared and Raman microscopies, we show that ATP purification increases the retention time of DNA-SWCNTs within cells while simultaneously enhancing the optical performance and stability of the engineered nanomaterial. Over a period of 6 h, we observe 45% brighter fluorescence intensity and no significant change in emission wavelength of ATP-purified DNA-SWCNTs relative to as-dispersed SWCNTs. These findings provide strong evidence of how cells differentially process engineered nanomaterials depending on their state of purification, lending to the future development of more robust and sensitive biosensors with desirable in vivo optical parameters using surfactant-based ATP systems with a subsequent exchange to biocompatible functionalization.

High Sensitivity Near-Infrared Imaging of Fluorescent Nanosensors

Biochemical processes are fast and occur on small-length scales, which makes them difficult to measure. Optical nanosensors based on single-wall carbon nanotubes (SWCNTs) are able to capture such dynamics. They fluoresce in the near-infrared (NIR, 850-1700 nm) tissue transparency window and the emission wavelength depends on their chirality. However, NIR imaging requires specialized indium gallium arsenide (InGaAs) cameras with a typically low resolution because the quantum yield of normal Si-based cameras rapidly decreases in the NIR. Here, an efficient one-step phase separation approach to isolate monochiral (6,4)-SWCNTs (880 nm emission) from mixed SWCNT samples is developed. It enables imaging them in the NIR with high-resolution standard Si-based cameras (>50× more pixels). (6,4)-SWCNTs modified with (GT)₁₀ -ssDNA become highly sensitive to the important neurotransmitter dopamine. These sensors are 1.7× brighter and 7.5× more sensitive and allow fast imaging (<50 ms). They enable high-resolution imaging of dopamine release from cells. Thus, the assembly of biosensors from (6,4)-SWCNTs combines the advantages of nanosensors working in the NIR with the sensitivity of (Si-based) cameras and enables broad usage of these nanomaterials.

Decoupling Individual Optical Nanosensor Responses Using a Spin-Coated Hydrogel Platform

Significant advances have been made in fields such as nanotechnology and biomedicine using the unique properties of single-walled carbon nanotubes (SWCNTs). Specifically, SWCNTs are used as near-infrared fluorescence sensors in the solution phase to detect a wide array of biologically relevant analytes. However, solution-based sensing has several limitations, including limited sensitivity and poor spatial resolution. We have therefore devised a new spin-coated poly(ethylene glycol) diacrylate (PEG-DA) hydrogel platform to examine individual DNA-functionalized SWCNTs (DNA-SWCNTs) in their native aqueous state and have subsequently used this platform to investigate the temporal modulations of each SWCNT in response to a model analyte. A strong surfactant, sodium deoxycholate (SDC), was chosen as the model analyte as it rapidly exchanges with DNA oligonucleotides on the SWCNT surface, modulating several optical properties of the SWCNTs and demonstrating multiparameter analyte detection. Upon addition of SDC, we observed time-dependent spectral modulations in the emission center wavelengths and peak intensities of the individual SWCNTs, indicative of a DNA-to-surfactant exchange process. Interestingly, we found that the modulations in the peak intensities, as determined by kinetic data, were significantly delayed when compared to their center wavelength counterparts, suggesting a potential decoupling of the response of these two spectral features. We used a 1-D diffusion model to relate the local SDC concentration to the spectral response of each SWCNT and created dose-response curves. The peak intensity shifts at a higher SDC concentration than the center wavelength, indicating a potential change in the conformation of the surfactant molecules adsorbed to the SWCNT sidewall after the initial exchange process. This platform allows for a unique single-molecule analysis technique that is significantly more sensitive and modifiable than utilizing SWCNTs in the solution phase.

Attachment of DNA-Wrapped Single-Walled Carbon Nanotubes (SWNTs) for a Micron-Sized Biosensor

We fabricated a micron-sized biodevice based on the near-infrared photoluminescence (PL) response of single-walled carbon nanotubes (SWNTs). Various biosensors using the unique optical responses of SWNTs have been proposed by many research groups. Most of these employed either colloidal suspensions of dispersed SWNTs or SWNT films on flat surfaces, such as electrodes. In this study, we attached DNA-wrapped SWNTs (DNA-SWNTs) to frustule (micron-sized nanoporous biosilica) surfaces, which were purified from cultured isolated diatoms. After the injection of an oxidant and a reductant, the SWNTs on the frustules showed prominent PL responses. This suggests that the biodevice functions as a micron-sized redox sensor. Frustules can be easily suspended in aqueous solutions because of their porous structures and can easily be collected as pellets by low-speed centrifugation. Thus, the removal of unbound SWNTs and the recovery of the fabricated DNA-SWNT frustules for reuse were achieved by gentle centrifugation. Our proposal for micron-sized SWNT biodevices would be helpful for various biological applications.

Distinguishing Antioxidant Molecules with Near-Infrared Photoluminescence of DNA-Wrapped Single-Walled Carbon Nanotubes

In this study, two biomolecule solutions were distinguished using the capacity difference in the near-infrared photoluminescence (PL) of single-walled carbon nanotubes (SWNTs). Biosensing techniques using sensitive responses of SWNTs have been intensively studied. When a small amount of an oxidant or reductant solution was injected into the SWNT suspensions, the PL intensity of the SWNTs is significantly changed. However, distinguishing between different molecules remains challenging. In this study, we comparably injected saponin and banana solutions, which are known antioxidant chemicals, into an SWNT suspension. The SWNTs were solubilized by wrapping them with DNA molecules. The results show that 69.1 and 155.2% increases of PL intensities of SWNTs were observed after injection of 20 and 59 μg/mL saponin solutions, respectively. Subsequently, the increase in PL was saturated. With the banana solution, 18.1 and 175.4% increases in PL intensities were observed with 20 and 59 μg/mL banana solutions, respectively. Based on these results, the two antioxidant molecules could be distinguished based on the different PL responses of the SWNTs. In addition, the much higher saturated PL intensities observed with the banana solution suggests that the banana solution increased the capacity of the PL increase for the same SWNT suspension. These results provide helpful information for establishing biosensing applications of SWNTs, particularly for distinguishing chemicals.

Aggregation Reduces Subcellular Localization and Cytotoxicity of Single-Walled Carbon Nanotubes

The non-covalent biomolecular functionalization of fluorescent single-walled carbon nanotubes (SWCNTs) has resulted in numerous in vitro and in vivo sensing and imaging applications due to many desirable optical properties. In these applications, it is generally presumed that pristine, singly dispersed SWCNTs interact with and enter live cells at the so-called nano-biointerface, for example, the cell membrane. Despite numerous fundamental studies published on this presumption, it is known that nanomaterials have the propensity to aggregate in protein-containing environments before ever contacting the nano-biointerface. Here, using DNA-functionalized SWCNTs with defined degrees of aggregation as well as near-infrared hyperspectral microscopy and toxicological assays, we show that despite equal rates of internalization, initially aggregated SWCNTs do not further accumulate within individual subcellular locations. In addition to subcellular accumulations, SWCNTs initially with a low degree of aggregation can induce significant deleterious effects in various long-term cytotoxicity and real-time proliferation assays, which are markedly different when compared to those of SWCNTs that are initially aggregated. These findings suggest the importance of the aggregation state as a critical component related to intracellular processing and toxicological response of engineered nanomaterials.

Hyperspectral Counting of Multiplexed Nanoparticle Emitters in Single Cells and Organelles

Nanomaterials are the subject of a range of biomedical, commercial, and environmental investigations involving measurements in living cells and tissues. Accurate quantification of nanomaterials, at the tissue, cell, and organelle levels, is often difficult, however, in part due to their inhomogeneity. Here, we propose a method that uses the distinct optical properties of a heterogeneous nanomaterial preparation in order to improve quantification at the single-cell and organelle level. We developed "hyperspectral counting", which employs diffraction-limited imaging via hyperspectral microscopy of a diverse set of fluorescent nanomaterials to estimate particle number counts in live cells and subcellular structures. A mathematical model was developed, and Monte Carlo simulations were employed, to improve the accuracy of these estimates, enabling quantification with single-cell and single-endosome resolution. We applied this nanometrology technique with single-walled carbon nanotubes and identified an upper limit of the rate of uptake into cells─approximately 3,000 nanotubes endocytosed within 30 min. In contrast, conventional region-of-interest counting results in a 230% undercount. The method identified significant heterogeneity and a broad non-Gaussian distribution of carbon nanotube uptake within cells. For example, while a particular cell contained an average of 1 nanotube per endosome, the heterogeneous distribution resulted in over 7 nanotubes localizing within some endosomes, substantially changing the accounting of subcellular nanoparticle concentration distributions. This work presents a method to quantify the cellular and subcellular concentrations of a heterogeneous carbon nanotube reference material, with implications for the nanotoxicology, drug/gene delivery, and nanosensor fields.