Biomembrane interactions of functionalized cryptophane-A: combined fluorescence and 129Xe NMR studies of a bimodal contrast agent

The detection methods currently available for protein aggregation in neurological diseases

Protein aggregation is a pathological feature in various neurodegenerative diseases and is thought to play a crucial role in the onset and progression of neurological disorders. This pathological phenomenon has attracted increasing attention from researchers, but the underlying mechanism has not been fully elucidated yet. Researchers are increasingly interested in identifying chemicals or methods that can effectively detect protein aggregation or maintain protein stability to prevent aggregation formation. To date, several methods are available for detecting protein aggregates, including fluorescence correlation spectroscopy, electron microscopy, and molecular detection methods. Unfortunately, there is still a lack of methods to observe protein aggregation in situ under a microscope. This article reviews the two main aspects of protein aggregation: the mechanisms and detection methods of protein aggregation. The aim is to provide clues for the development of new methods to study this pathological phenomenon.

Improving HyperCEST performance by favorable xenon exchange conditions in liposomal nanocarriers

MRI reporters that combine signal enhancement from saturation transfer with hyperpolarized ¹²⁹ Xe show nanomolar detection sensitivity for in vitro studies. However, they need further improvement for accelerated CEST build-up that sufficiently dominates the intrinsic loss of hyperpolarization under in vivo conditions. This study introduces liposomes with a HyperCEST-active lipopeptide to enhance the efficiency of a well known Xe host, CrA-ma, with medium Xe exchange kinetics in aqueous environment, by two orders of magnitude. The depolarization time for constant saturation power but increasing saturation time is used as a comparative measure to rank different nanocarrier formulations. A variable cage load illustrates that the available CEST sites should be well distributed throughout the nanocarriers to avoid inefficiency from back exchange. For a liposome loading with only 2 mol% CrA-lipopeptide, the higher exchange kinetics allowed us to work even with 17-fold lower saturation power than for CrA-ma itself to achieve significant image contrast with ¹²⁹ Xe. Overall, this study illustrates the wide parameter space that is now available when incorporating CrA-labelled lipopeptides into liposomal carriers.

Towards Probing Conformational States of Y2 Receptor Using Hyperpolarized 129Xe NMR

G protein-coupled receptors can adopt many different conformational states, each of them exhibiting different restraints towards downstream signaling pathways. One promising strategy to identify and quantify this conformational landscape is to introduce a cysteine at a receptor site sensitive to different states and label this cysteine with a probe for detection. Here, the application of NMR of hyperpolarized ¹²⁹Xe for the detection of the conformational states of human neuropeptide Y2 receptor is introduced. The xenon trapping cage molecule cryptophane-A attached to a cysteine in extracellular loop 2 of the receptor facilitates chemical exchange saturation transfer experiments without and in the presence of native ligand neuropeptide Y. High-quality spectra indicative of structural states of the receptor-cage conjugate were obtained. Specifically, five signals could be assigned to the conjugate in the apo form. After the addition of NPY, one additional signal and subtle modifications in the persisting signals could be detected. The correlation of the spectroscopic signals and structural states was achieved with molecular dynamics simulations, suggesting frequent contact between the xenon trapping cage and the receptor surface but a preferred interaction with the bound ligand.

In vivo hyperCEST imaging: Experimental considerations for a reliable contrast

PURPOSE

HyperCEST contrast relies on the reduction of the solvent signal after selective saturation of the solute magnetization. The scope of this work is to outline the experimental conditions needed to obtain a reliable hyperCEST contrast in vivo, where the "solvent" signal (ie, the dissolved-phase signal) may change over time due to the increase in xenon (Xe) accumulation into tissue.

METHODS

Hyperpolarized ¹²⁹ Xe was delivered to mice at a constant volume and rate using a mechanical ventilator, which triggered the saturation, excitation, and acquisition of the MR signal during the exhale phase of the breath cycle-either every breath or every 2, 3, or 4 breaths. Serial Z-spectra and hyperCEST images were acquired before and after a bolus injection of cucurbit[6]uril to assess possible signal fluctuations and instabilities.

RESULTS

The intensity of the dissolved-phase Xe signal was observed to first increase immediately after the beginning of the hyperpolarized gas inhalation and NMR acquisition, and then decrease before reaching a steady-state condition. Once a steady-state dissolved-phase magnetization was established, a reliable hyperCEST contrast, exceeding 40% signal reduction, was observed.

CONCLUSION

A reliable hyperCEST contrast can only be obtained after establishing a steady-state dissolved phase ¹²⁹ Xe magnetization. Under stable physiological conditions, a steady-state dissolved-phase Xe magnetization is only achieved after a series of Xe inhalations and RF excitations, and it requires synchronization of the breathing rate with the MR acquisition.

Identification of extracellular nanoparticle subsets by nuclear magnetic resonance

Exosomes are a subset of secreted lipid envelope-encapsulated extracellular vesicles (EVs) of 50-150 nm diameter that can transfer cargo from donor to acceptor cells. In the current purification protocols of exosomes, many smaller and larger nanoparticles such as lipoproteins, exomers and microvesicles are typically co-isolated as well. Particle size distribution is one important characteristics of EV samples, as it reflects the cellular origin of EVs and the purity of the isolation. However, most of the physicochemical analytical methods today cannot illustrate the smallest exosomes and other small particles like the exomers. Here, we demonstrate that diffusion ordered spectroscopy (DOSY) nuclear magnetic resonance (NMR) method enables the determination of a very broad distribution of extracellular nanoparticles, ranging from 1 to 500 nm. The range covers sizes of all particles included in EV samples after isolation. The method is non-invasive, as it does not require any labelling or other chemical modification. We investigated EVs secreted from milk as well as embryonic kidney and renal carcinoma cells. Western blot analysis and immuno-electron microscopy confirmed expression of exosomal markers such as ALIX, TSG101, CD81, CD9, and CD63 in the EV samples. In addition to the larger particles observed by nanoparticle tracking analysis (NTA) in the range of 70-500 nm, the DOSY distributions include a significant number of smaller particles in the range of 10-70 nm, which are visible also in transmission electron microscopy images but invisible in NTA. Furthermore, we demonstrate that hyperpolarized chemical exchange saturation transfer (Hyper-CEST) with 129Xe NMR indicates also the existence of smaller and larger nanoparticles in the EV samples, providing also additional support for DOSY results. The method implies also that the Xe exchange is significantly faster in the EV pool than in the lipoprotein/exomer pool.

Molecular Sensing with Host Systems for Hyperpolarized 129Xe

Hyperpolarized noble gases have been used early on in applications for sensitivity enhanced NMR. 129Xe has been explored for various applications because it can be used beyond the gas-driven examination of void spaces. Its solubility in aqueous solutions and its affinity for hydrophobic binding pockets allows "functionalization" through combination with host structures that bind one or multiple gas atoms. Moreover, the transient nature of gas binding in such hosts allows the combination with another signal enhancement technique, namely chemical exchange saturation transfer (CEST). Different systems have been investigated for implementing various types of so-called Xe biosensors where the gas binds to a targeted host to address molecular markers or to sense biophysical parameters. This review summarizes developments in biosensor design and synthesis for achieving molecular sensing with NMR at unprecedented sensitivity. Aspects regarding Xe exchange kinetics and chemical engineering of various classes of hosts for an efficient build-up of the CEST effect will also be discussed as well as the cavity design of host molecules to identify a pool of bound Xe. The concept is presented in the broader context of reporter design with insights from other modalities that are helpful for advancing the field of Xe biosensors.

Interaction of Cucurbit[7]uril With Protease Substrates: Application to Nanosecond Time-Resolved Fluorescence Assays

We report the use of the macrocyclic host cucurbit[7]uril (CB7) as a supramolecular additive in nanosecond time-resolved fluorescence (Nano-TRF) assays for proteases to enhance the discrimination of substrates and products and, thereby, the sensitivity. A peptide substrate was labeled with 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) as a long-lived (>300 ns) fluorescent probe and 3-nitrotyrosine was established as a non-fluorescent fluorescence resonance energy transfer (FRET) acceptor that acts as a “dark quencher.” The substrate was cleaved by the model proteases trypsin and chymotrypsin and the effects of the addition of CB7 to the enzyme assay mixture were investigated in detail using UV/VIS absorption as well as steady-state and time-resolved fluorescence spectroscopy. This also allowed us to identify the DBO and nitrotyrosine residues as preferential binding sites for CB7 and suggested a hairpin conformation of the peptide, in which the guanidinium side chain of an arginine residue is additionally bound to a vacant ureido rim of one of the CB7 hosts.

Functionalized Lipopeptide Micelles as Highly Efficient NMR Depolarization Seed Points for Targeted Cell Labelling in Xenon MRI

Improving diagnostic imaging and therapy by targeted compound delivery to pathological areas and across biological barriers is of urgent need. A lipopeptide, P-CrA-A2, composed of a highly cationic peptide sequence (A2), an N-terminally attached palmitoyl chain (P) and cryptophane molecule (CrA) for preferred uptake into blood-brain barrier (BBB) capillary endothelial cells, was generated. CrA allows reversible binding of Xe for NMR detection with hyperpolarized nuclei. The lipopeptide forms size-optimized micelles with a diameter of about 11 nm at low micromolar concentration. Their high local CrA payload has a strong and switchable impact on the bulk magnetization through Hyper-CEST detection. Covalent fixation of CrA does not impede micelle formation and does not hamper its host functionality but simplifies Xe access to hosts for inducing saturation transfer. Xe Hyper-CEST magnetic resonance imaging (MRI) allows for distinguishing BBB endothelial cells from control aortic endothelial cells, and the small micelle volume with a sevenfold improved CrA-loading density compared to liposomal carriers allows preferred cell labelling with a minimally invasive volume (≈16 000-fold more efficient than ¹⁹ F cell labelling). Thus, these nanoscopic particles combine selectivity for human brain capillary endothelial cells with great sensitivity of Xe Hyper-CEST MRI and might be a potential MRI tool in brain diagnostics.

Supramolecular Chemistry in the Biomembrane

The combination of supramolecular functional systems with biomolecular chemistry has been a fruitful exercise for decades, leading to a greater understanding of biomolecules and to a great variety of applications, for example, in drug delivery and sensing. Within these developments, the phospholipid bilayer membrane, surrounding live cells, with all its functions has also intrigued supramolecular chemists. Herein, recent efforts from the supramolecular chemistry community to mimic natural functions of lipid membranes, such as sensing, molecular recognition, membrane fusion, signal transduction, and gated transport, are reviewed.

Nanoparticle-Based Contrast Agents for 129Xe HyperCEST NMR and MRI Applications

Spin hyperpolarization techniques have enabled important advancements in preclinical and clinical MRI applications to overcome the intrinsic low sensitivity of nuclear magnetic resonance. Functionalized xenon biosensors represent one of these approaches. They combine two amplification strategies, namely, spin exchange optical pumping (SEOP) and chemical exchange saturation transfer (CEST). The latter one requires host structures that reversibly bind the hyperpolarized noble gas. Different nanoparticle approaches have been implemented and have enabled molecular MRI with 129Xe at unprecedented sensitivity. This review gives an overview of the Xe biosensor concept, particularly how different nanoparticles address various critical aspects of gas binding and exchange, spectral dispersion for multiplexing, and targeted reporter delivery. As this concept is emerging into preclinical applications, comprehensive sensor design will be indispensable in translating the outstanding sensitivity potential into biomedical molecular imaging applications.

In vivo detection of cucurbit[6]uril, a hyperpolarized xenon contrast agent for a xenon magnetic resonance imaging biosensor

The Hyperpolarized gas Chemical Exchange Saturation Transfer (HyperCEST) Magnetic Resonance (MR) technique has the potential to increase the sensitivity of a hyperpolarized xenon-129 MRI contrast agent. Signal enhancement is accomplished by selectively depolarizing the xenon within a cage molecule which, upon exchange, reduces the signal in the dissolved phase pool. Herein we demonstrate the in vivo detection of the cucurbit[6]uril (CB6) contrast agent within the vasculature of a living rat. Our work may be used as a stepping stone towards using the HyperCEST technique as a molecular imaging modality.

NMR Hyperpolarization Techniques of Gases

Nuclear spin polarization can be significantly increased through the process of hyperpolarization, leading to an increase in the sensitivity of nuclear magnetic resonance (NMR) experiments by 4-8 orders of magnitude. Hyperpolarized gases, unlike liquids and solids, can often be readily separated and purified from the compounds used to mediate the hyperpolarization processes. These pure hyperpolarized gases enabled many novel MRI applications including the visualization of void spaces, imaging of lung function, and remote detection. Additionally, hyperpolarized gases can be dissolved in liquids and can be used as sensitive molecular probes and reporters. This Minireview covers the fundamentals of the preparation of hyperpolarized gases and focuses on selected applications of interest to biomedicine and materials science.

Simple and rapid quantification of phospholipids for supramolecular membrane transport assays

We introduce a simple ¹H NMR method for quantification of the phospholipid content of liposomes.

Supramolecular Assays for Mapping Enzyme Activity by Displacement-Triggered Change in Hyperpolarized (129)Xe Magnetization Transfer NMR Spectroscopy

Reversibly bound Xe is a sensitive NMR and MRI reporter with its resonance frequency being influenced by the chemical environment of the host. Molecular imaging of enzyme activity presents a promising approach for disease identification, but current Xe biosensing concepts are limited since substrate conversion typically has little impact on the chemical shift of Xe inside tailored cavities. Herein, we exploit the ability of the product of the enzymatic reaction to bind itself to the macrocyclic hosts CB6 and CB7 and thereby displace Xe. We demonstrate the suitability of this method to map areas of enzyme activity through changes in magnetization transfer with hyperpolarized Xe under different saturation scenarios.

Cucurbit[6]uril is an ultrasensitive (129)Xe NMR contrast agent

A lack of molecular contrast agents has slowed the application of ultrasensitive hyperpolarized (129)Xe NMR methods. Here, we report that commercially available cucurbit[6]uril (CB[6]) undergoes rapid xenon exchange kinetics at 300 K, and is detectable by Hyper-CEST NMR at 1.8 pM in PBS and at 1 μM in human plasma where many molecules, including polyamines, can compete with xenon for CB[6] binding.

A "Smart" ¹²⁸Xe NMR Biosensor for pH-Dependent Cell Labeling

Here we present a "smart" xenon-129 NMR biosensor that undergoes a peptide conformational change and labels cells in acidic environments. To a cryptophane host molecule with high Xe affinity, we conjugated a 30mer EALA-repeat peptide that is α-helical at pH 5.5 and disordered at pH 7.5. The (129)Xe NMR chemical shift at room temperature was strongly pH-dependent (Δδ = 3.4 ppm): δ = 64.2 ppm at pH 7.5 vs δ = 67.6 ppm at pH 5.5, where Trp(peptide)-cryptophane interactions were evidenced by Trp fluorescence quenching. Using hyper-CEST NMR, we probed peptidocryptophane detection limits at low-picomolar (10(-11) M) concentration, which compares favorably to other NMR pH reporters at 10(-2)-10(-3) M. Finally, in biosensor-HeLa cell solutions, peptide-cell membrane insertion at pH 5.5 generated a 13.4 ppm downfield cryptophane-(129)Xe NMR chemical shift relative to pH 7.5 studies. This highlights new uses for (129)Xe as an ultrasensitive probe of peptide structure and function, along with potential applications for pH-dependent cell labeling in cancer diagnosis and treatment.

Nile red dye in aqueous surfactant and micellar solution

The solubilization behavior of nile red dye in aqueous surfactant and micellar solutions was studied by optical spectroscopic techniques, dynamic light scattering, and atomic force microscopy. Nile red exhibits considerable absorption in the submicellar concentration region. When dispersed in aqueous surfactant and/or micellar solution, nile red molecules tend to form nonemissive dimers and/or H-type aggregates through π-π stacking interactions. This phenomenon may limit the use of nile red in solubilization studies. In the presence of ionic SDS and CTAB micelles, the solubilization of nile red appears to take place primarily at the charged micellar surface within the interfacial region. Similarly, spectra in micellar solution of nonionic Triton X-100 revealed that nile red dye penetrates the hydrophilic, interfacial poly(oxyethylene) region of the micelles but cannot reach the hydrophobic, innermost core. Our results therefore suggest that nile red dye must be chosen carefully when probing (micellar) hydrophobic environments and (micro)domains.

Brain endothelial cell targeting via a peptide-functionalized liposomal carrier for xenon hyper-CEST MRI

A nanoparticulate carrier system is used to efficiently deliver a contrast agent for highly sensitive xenon Hyper-CEST MRI. The carrier system not only improves the biocompatibility and solubility of the contrast agent, it also allows selective cell targeting as demonstrated by the discrimination of human brain capillary and aortic endothelial cells.

Depolarization Laplace transform analysis of exchangeable hyperpolarized ¹²⁹Xe for detecting ordering phases and cholesterol content of biomembrane models

We present a highly sensitive nuclear-magnetic resonance technique to study membrane dynamics that combines the temporary encapsulation of spin-hyperpolarized xenon ((129)Xe) atoms in cryptophane-A-monoacid (CrAma) and their indirect detection through chemical exchange saturation transfer. Radiofrequency-labeled Xe@CrAma complexes exhibit characteristic differences in chemical exchange saturation transfer-driven depolarization when interacting with binary membrane models composed of different molecular ratios of DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) and POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine). The method is also applied to mixtures of cholesterol and POPC. The existence of domains that fluctuate in cluster size in DPPC/POPC models at a high (75-98%) DPPC content induces up to a fivefold increase in spin depolarization time τ at 297 K. In POPC/cholesterol model membranes, the parameter τ depends linearly on the cholesterol content at 310 K and allows us to determine the cholesterol content with an accuracy of at least 5%.

Observing and preventing rubidium runaway in a direct-infusion xenon-spin hyperpolarizer optimized for high-resolution hyper-CEST (chemical exchange saturation transfer using hyperpolarized nuclei) NMR

Xenon is well known to undergo host-guest interactions with proteins and synthetic molecules. As xenon can also be hyperpolarized by spin exchange optical pumping, allowing the investigation of highly dilute systems, it makes an ideal nuclear magnetic resonance probe for such host molecules. The utility of xenon as a probe can be further improved using Chemical Exchange Saturation Transfer using hyperpolarized nuclei (Hyper-CEST), but for highly accurate experiments requires a polarizer and xenon infusion system optimized for such measurements. We present the design of a hyperpolarizer and xenon infusion system specifically designed to meet the requirements of Hyper-CEST measurements. One key element of this design is preventing rubidium runaway, a chain reaction induced by laser heating that prevents efficient utilization of high photon densities. Using thermocouples positioned along the pumping cell we identify the sources of heating and conditions for rubidium runaway to occur. We then demonstrate the effectiveness of actively cooling the optical cell to prevent rubidium runaway in a compact setup. This results in a 2-3-fold higher polarization than without cooling, allowing us to achieve a polarization of 25% at continuous flow rates of 9 ml/min of (129)Xe. The simplicity of this design also allows it to be retrofitted to many existing polarizers. Combined with a direction infusion system that reduces shot-to-shot noise down to 0.56% we have captured Hyper-CEST spectra in unprecedented detail, allowing us to completely resolve peaks separated by just 1.62 ppm. Due to its high polarization and excellent stability, our design allows the comparison of underlying theories of host-guest systems with experiment at low concentrations, something extremely difficult with previous polarizers.

Development of an antibody-based, modular biosensor for 129Xe NMR molecular imaging of cells at nanomolar concentrations

Magnetic resonance imaging (MRI) is seriously limited when aiming for visualization of targeted contrast agents. Images are reconstructed from the weak diamagnetic properties of the sample and require an abundant molecule like water as the reporter. Micromolar to millimolar concentrations of conventional contrast agents are needed to generate image contrast, thus excluding many molecular markers as potential targets. To address this limitation, we developed and characterized a functional xenon NMR biosensor that can identify a specific cell surface marker by targeted (129)Xe MRI. Cells expressing the cell surface protein CD14 can be spatially distinguished from control cells with incorporation of as little as 20 nM of the xenon MRI readout unit, cryptophane-A. Cryptophane-A serves as a chemical host for hyperpolarized nuclei and facilitates the sensitivity enhancement achieved by xenon MRI. Although this paper describes the application of a CD14-specific biosensor, the construct has been designed in a versatile, modular fashion. This allows for quick and easy adaptation of the biosensor to any cell surface target for which there is a specific antibody. In addition, the modular design facilitates the creation of a multifunctional probe that incorporates readout modules for different detection methods, such as fluorescence, to complement the primary MRI readout. This modular antibody-based approach not only offers a practical technique with which to screen targets, but one which can be readily applied as the xenon MRI field moves closer to molecular imaging applications in vivo.

Bacterial spore detection and analysis using hyperpolarized 129Xe chemical exchange saturation transfer (Hyper-CEST) NMR

Previously, we reported hyperpolarized ¹²⁹Xe chemical exchange saturation transfer (Hyper-CEST) NMR techniques for the ultrasensitive (i.e., 1 picomolar) detection of xenon host molecules known as cryptophane. Here, we demonstrate a more general role for Hyper-CEST NMR as a spectroscopic method for probing nanoporous structures, without the requirement for cryptophane or engineered xenon-binding sites. Hyper-CEST ¹²⁹Xe NMR spectroscopy was employed to detect Bacillus anthracis and Bacillus subtilis spores in solution, and interrogate the layers that comprise their structures. ¹²⁹Xe-spore samples were selectively irradiated with radiofrequency pulses; the depolarized ¹²⁹Xe returned to aqueous solution and depleted the ¹²⁹Xe-water signal, providing measurable contrast. Removal of the outermost spore layers in B. anthracis and B. subtilis (the exosporium and coat, respectively) enhanced ¹²⁹Xe exchange with the spore interior. Notably, the spores were invisible to hyperpolarized ¹²⁹Xe NMR direct detection methods, highlighting the lack of high-affinity xenon-binding sites, and the potential for extending Hyper-CEST NMR structural analysis to other biological and synthetic nanoporous structures.

Ultrafast Z-Spectroscopy for 129Xe NMR-Based Sensors

When working with hyperpolarized species, it is often difficult to maintain a stable level of magnetization over consecutive experiments, which renders their detection at the trace level cumbersome, even when combined with chemical exchange saturation transfer (CEST). We report herein the use of ultra-fast Z-spectroscopy as a powerful means to detect low concentrations of 129Xe NMR-based sensors and to measure the in-out xenon exchange. Modifications of the original sequence enable a multiplexed detection of several sensors, as well as the extraction of the exchange buildup rate constant in a single-shot fashion.

Cell tracking with caged xenon: using cryptophanes as MRI reporters upon cellular internalization

Caged xenon has great potential in overcoming sensitivity limitations for solution-state NMR detection of dilute molecules. However, no application of such a system as a magnetic resonance imaging (MRI) contrast agent has yet been performed with live cells. We demonstrate MRI localization of cells labeled with caged xenon in a packed-bed bioreactor working under perfusion with hyperpolarized-xenon-saturated medium. Xenon hosts enable NMR/MRI experiments with switchable contrast and selectivity for cell-associated versus unbound cages. We present MR images with 10(3) -fold sensitivity enhancement for cell-internalized, dual-mode (fluorescence/MRI) xenon hosts at low micromolar concentrations. Our results illustrate the capability of functionalized xenon to act as a highly sensitive cell tracer for MRI detection even without signal averaging. The method will bridge the challenging gap for translation to in vivo studies for the optimization of targeted biosensors and their multiplexing applications.

Nile-Red-nanoclay hybrids: red emissive optical probes for use in aqueous dispersion

Water-dispersible and (bio)functionalizable nanoclays have a considerable potential as inexpensive carriers for organic molecules like drugs and fluorophores. Aiming at simple design strategies for red-emissive optical probes for the life sciences from commercial precursors with minimum synthetic effort, we systematically studied the dye loading behavior and stability of differently functionalized laponites. Here, we present a comprehensive study of the absorption and emission properties of the red emissive hydrophobic and neutral dye Nile Red, a well-known polarity probe, which is almost insoluble and nonemissive in water. Adsorption of this probe onto disk-shaped nanoclays was studied in aqueous dispersion as function of dye concentration, in the absence and presence of the cationic surfactant cetyltrimethylammonium bromide (CTAB) assisting dye loading, and as a function of pH. This laponite loading strategy yields strongly fluorescent nanoclay suspensions with a fluorescence quantum yield of 0.34 at low dye loading concentration. The dye concentration-, CTAB-, and pH-dependent absorption, fluorescence emission, and fluorescence excitation spectra of the Nile-Red-nanoclay suspensions suggest the formation of several Nile Red species including emissive Nile Red monomers facing a polar environment, nonemissive H-type dimers, and protonated Nile Red molecules that are also nonfluorescent. Formation of all nonemissive Nile Red species could be suppressed by modification of the laponite with CTAB. This underlines the great potential of properly modified and functionalized laponite nanodisks as platform for optical probes with drug delivery capacities, for example, for tumor and therapy imaging. Moreover, comparison of the Nile Red dimer absorption spectra with absorption spectra of previously studied Nile Red aggregates in dendrimer systems and micelles and other literature systems reveals a considerable dependence of the dimer absorption band on microenvironment polarity which has not yet been reported so far for H-type dye aggregates.