Cyclodextrins as eminent constituents in nanoarchitectonics for drug delivery systems

Cyclodextrins have been widely employed for drug delivery systems (DDSs) in which drugs are selectively delivered to a target site in the body. Recent interest has been focused on the construction of cyclodextrin-based nanoarchitectures that show sophisticated DDS functions. These nanoarchitectures are precisely fabricated based on three important features of cyclodextrins, namely (1) the preorganized three-dimensional molecular structure of nanometer size, (2) the easy chemical modification to introduce functional groups, and (3) the formation of dynamic inclusion complexes with various guests in water. With the use of photoirradiation, drugs are released from cyclodextrin-based nanoarchitectures at designated timing. Alternatively, therapeutic nucleic acids are stably protected in the nanoarchitectures and delivered to the target site. The efficient delivery of the CRISPR-Cas9 system for gene editing was also successful. Even more complicated nanoarchitectures can be designed for sophisticated DDSs. Cyclodextrin-based nanoarchitectures are highly promising for future applications in medicine, pharmaceutics, and other relevant fields.

Lanthanide-Doped Upconversion Nanoparticles: Exploring A Treasure Trove of NIR-Mediated Emerging Applications

Lanthanide-doped upconversion nanoparticles (UCNPs) possess the remarkable ability to convert multiple near-infrared (NIR) photons into higher energy ultraviolet-visible (UV-vis) photons, making them a prime candidate for several advanced applications within the realm of nanotechnology. Compared to traditional organic fluorophores and quantum dots (QDs), UCNPs possess narrower emission bands (fwhm of 10-50 nm), large anti-Stokes shifts, low toxicity, high chemical stability, and resistance to photobleaching and blinking. In addition, unlike UV-vis excitation, NIR excitation is nondestructive at lower power intensities and has high tissue penetration depths (up to 2 mm) with low autofluorescence and scattering. Together, these properties make UCNPs exceedingly favored for advanced bioanalytical and theranostic applications, where these systems have been well-explored. UCNPs are also well-suited for bioimaging, optically modulating chemistries, forensic science, and other state-of-the-art research applications. In this review, an up-to-date account of emerging applications in UCNP research, beyond bioanalytical and theranostics, are presented including optogenetics, super-resolution imaging, encoded barcodes, fingerprinting, NIR vision, UCNP-assisted photochemical manipulations, optical tweezers, 3D printing, lasing, NIR-II imaging, UCNP-molecule nanohybrids, and UCNP-based persistent luminescent nanocrystals.

Light Responsiveness and Assembly of Arylazopyrazole-Based Surfactants in Neat and Mixed CTAB Micelles

The self-assembly of an arylazopyrazole-based photosurfactant (PS), based on cetyltrimethylammonium bromide (CTAB), and its mixed micelle formation with CTAB in aqueous solution was investigated by small angle neutron and X-ray scattering (SANS/SAXS) and UV-vis absorption spectroscopy. Upon UV light exposure, PS photoisomerizes from E-PS (trans) to Z-PS (cis), which transforms oblate ellipsoidal micelles into smaller, spherical micelles with larger shell thickness. Doping PS with CTAB resulted in mixed micelle formation at all stoichiometries and conditions investigated; employing selectively deuterated PS, a monotonic variation in scattering length density and dimensions of the micellar core and shell is observed for all contrasts. The concentration- and irradiance-dependence of the E to Z configurational transition was established in both neat and mixed micelles. A liposome dye release assay establishes the enhanced efficacy of photosurfactants at membrane disruption, with E-PS exhibiting a 4-fold and Z-PS a 10-fold increase in fluorescence signal with respect to pure CTAB. Our findings pave the way for external triggering and modulation of the wide range of CTAB-based biomedical and material applications.

Fully Symmetric Cyclodextrin Polycarboxylates: How to Determine Reliable Protonation Constants from NMR Titration Data

Acid-base properties of cyclodextrins (CDs), persubstituted at C-6 by 3-mercaptopropionic acid, sualphadex (Suα-CD), subetadex (Suβ-CD) and sugammadex (Suγ-CD, the antidote of neuromuscular blocking steroids) were studied by ¹H NMR-pH titrations. For each CD, the severe overlap in protonation steps prevented the calculation of macroscopic pK_a values using the standard data fitting model. Considering the full symmetry of polycarboxylate structures, we reduced the number of unknown NMR parameters in the "Q-fitting" or the novel "equidistant macroscopic" evaluation approaches. These models already provided pK_a values, but some of them proved to be physically unrealistic, deceptively suggesting cooperativity in carboxylate protonations. The latter problem could be circumvented by adapting the microscopic site-binding (cluster expansion) model by Borkovec, which applies pairwise interactivity parameters to quantify the mutual basicity-decreasing effect of carboxylate protonations. Surprisingly, only a single averaged interactivity parameter could be calculated reliably besides the carboxylate 'core' microconstant for each CD derivative. The speciation of protonation isomers hence could not be resolved, but the optimized microscopic basicity parameters could be converted to the following sets of macroscopic pK_a values: 3.84, 4.35, 4.81, 5.31, 5.78, 6.28 for Suα-CD; 3.82, 4.31, 4.73, 5.18, 5.64, 6.06, 6.54 for Suβ-CD and 3.83, 4.28, 4.65, 5.03, 5.43, 5.81, 6.18, 6.64 for Suγ-CD. The pH-dependent charge of these compounds can now be accurately calculated, in support of designing new analytical methods to exploit their charge-dependent molecular recognition such as in cyclodextrin-aided chiral capillary electrophoresis.

Photon Upconversion Systems Based on Triplet-Triplet Annihilation as Photosensitizers for Chemical Transformations

Photon upconversion (UC) based on triplet-triplet annihilation (TTA) is considered one of the most attractive methodologies for switching wavelengths from lower to higher energy. This two-photon process, which requires the involvement of a bimolecular system, has been widely used in numerous fields such as bioimaging, solar cells, displays, drug delivery, and so on. In the last years, we have witnessed the harnessing of this concept by the organic community who have developed new strategies for synthetic purposes. Interestingly, the generation of high-energetic species by this phenomenon has provided the opportunity not only to photoredox activate compounds with high-energy demanding bonds, expanding the reactivity window that lies outside the energy window of the initial irradiation wavelength, but also to sensitized conventional photocatalysts through energy transfer processes even employing infrared irradiation. Herein, an overview of the principal examples found in literature is described where TTA-UC systems are found to be suitable photosensitizers for several chemical transformations.

Highly effective gene delivery based on cyclodextrin multivalent assembly in target cancer cells

A supramolecular assembly based on cyclodextrins for highly effective gene delivery responded to NIR light and reductase in targeted cancer cells.

Photo-Responsive Self-Assembly of Plasmonic Magnetic Janus Nanoparticles

Stimuli-responsive self-assembly of nanoparticles is a versatile approach for the bottom-up fabrication of adaptive and functional nanomaterials. For this purpose, anisotropic building blocks are of particular importance due to the unique shapes and structures that can be obtained upon self-assembly. Here, we demonstrate the photo-responsive self-assembly of plasmonic magnetic "dumbbell" Janus nanoparticles (Au-Fe₃O₄) via the host-guest interaction of the supramolecular host cyclodextrin and the molecular photoswitch arylazopyrazole. We developed efficient ligand exchange procedures that enable the introduction of functional ligands, respectively, to the surface of the gold or magnetite core of the dumbbell. Our results indicate that distinct nanoparticle superstructures arise in aqueous solutions if nanoparticle aggregation is crosslinker-induced or self-induced and that the reversible formation and fragmentation of the superstructures can be modulated with light.

Simultaneous Incorporation of Two Types of Azo-Groups in the Side Chains of a Conjugated D-A Polymer for Logic Control of the Semiconducting Performance by Light Irradiation

A new design strategy for photoresponsive semiconducting polymer with tri-stable semiconducting states is reported by simultaneous incorporation of tetra-ortho-methoxy-substituted azobenzene (mAzo) and arylazopyrazole (pAzo) in the side chains. The trans-to-cis transformations for mAzo and pAzo groups can sequentially occur within the polymer thin film after sequential 560 and 365 nm light irradiation. Remarkably, the trans-cis isomerization of mAzo and pAzo groups can modulate the thin film crystallinity. Accordingly, the performances of the resulting field-effect transistors (FETs) can be reversibly modulated, leading to tri-stable semiconducting states after sequential 560, 365, and 470 nm light irradiation. Therefore, the device performance can be logically controlled by light irradiation at three different wavelengths. In addition, with light irradiation and device current as the input and output signals, the three-value logic gate by using single FET device can be successfully mimicked.

A Conjugated Polymer Containing Arylazopyrazole Units in the Side Chains for Field-Effect Transistors Optically Tunable by Near Infra-Red Light

Optically tunable field-effect transistors (FETs) with near infra-red (NIR) light show promising applications in various areas. Now, arylazopyrazole groups are incorporated in the side chains of a semiconducting donor-acceptor (D-A) polymer. The cis-trans interconversion of the arylazopyrazole can be controlled by 980 nm and 808 nm NIR light irradiation, by utilizing NaYF₄ :Yb,Tm upconversion nanoparticles and the photothermal effect of conjugated D-A polymers, respectively. This reversible transformation affects the interchain packing of the polymer thin film, which in turn reversibly tunes the semiconducting properties of the FETs by the successive 980 nm and 808 nm light irradiation. The resultant FETs display fast response to NIR light, good resistance to photofatigue, and stability in storage for up to 120 days. These unique features will be useful in future memory and bioelectronic wearable devices.

Wholly Visible-Light-Responsive Host-Guest Supramolecular Gels Based on Methoxy Azobenzene and β-Cyclodextrin Dimers

Much attention has been paid to construct photoresponsive host-guest supramolecular gels; however, red-shifting the responsive wavelength remains a formidable challenge. Here, a wholly visible-light-responsive supramolecular gel was fabricated through the host-guest interaction between a β-cyclodextrin (β-CD) dimer and a tetra-ortho-methoxy-substituted azobenzene (mAzo) dimer (binary gelator) in DMSO/H₂O (V/V = 8/2). The minimum gelation concentration of the low-molecular-weight binary gelator was 6 wt % measured via the tube inversion method. The substituted methoxy groups shifted the responsive wavelengths of trans-mAzo and cis-mAzo to the green and blue light regions, respectively. The host-guest interaction between mAzo and β-CD as the driving force for gelation was confirmed using the ¹H-NMR and 2D ¹H NOESY spectra. The supramolecular gel showed good self-supporting ability with a storage modulus higher than 10⁴ Pa. The release of Rhodamine B loaded in the gel as a model drug could be controlled by green light irradiation. We envisioned the potential applications of the wholly visible-light-responsive supramolecular compounds ranging from biomedical materials to smart materials.

The Many Ways to Assemble Nanoparticles Using Light

The ability to reversibly assemble nanoparticles using light is both fundamentally interesting and important for applications ranging from reversible data storage to controlled drug delivery. Here, the diverse approaches that have so far been developed to control the self-assembly of nanoparticles using light are reviewed and compared. These approaches include functionalizing nanoparticles with monolayers of photoresponsive molecules, placing them in photoresponsive media capable of reversibly protonating the particles under light, and decorating plasmonic nanoparticles with thermoresponsive polymers, to name just a few. The applicability of these methods to larger, micrometer-sized particles is also discussed. Finally, several perspectives on further developments in the field are offered.

Photoresponsive Zn2+-specific metallohydrogels coassembled from imidazole containing phenylalanine and arylazopyrazole derivatives

Stimuli-responsive supramolecular gels and metallogels have been widely explored in the past decade, but the fabrication of metallogels with reversible photoresponsive properties remains largely unexplored.

Cyclodextrin-Based Multistimuli-Responsive Supramolecular Assemblies and Their Biological Functions

Cyclodextrins (CDs), which are a class of cyclic oligosaccharides extracted from the enzymatic degradation of starch, are often utilized in molecular recognition and assembly constructs, primarily via host-guest interactions in water. In this review, recent progress in CD-based supramolecular nanoassemblies that are sensitive to chemical, biological, and physical stimuli is updated and reviewed, and intriguing examples of the biological functions of these nanoassemblies are presented, including pH- and redox-responsive drug and gene delivery, enzyme-activated specific cargo release, photoswitchable morphological interconversion, microtubular aggregation, and cell-cell communication, as well as a geomagnetism-controlled nanosystem for the suppression of tumor invasion and metastasis. Moreover, future perspectives and challenges in the fabrication of intelligent CD-based biofunctional materials are also discussed at the end of this review, which is expected to promote the translational development of these nanomaterials in the biomedical field.

Reversible end-to-end assembly of selectively functionalized gold nanorods by light-responsive arylazopyrazole-cyclodextrin interaction

We propose a two-step ligand exchange for the selective end-functionalization of gold nanorods (AuNR) by thiolated cyclodextrin (CD) host molecules. As a result of the complete removal of the precursor capping agent cetyltrimethylammonium bromide (CTAB) by a tetraethylene glycol derivative, competitive binding to the host cavity was prevented, and reversible, light-responsive assembly and disassembly of the AuNR could be induced by host-guest interaction of CD on the nanorods and a photoswitchable arylazopyrazole cross-linker in aqueous solution. The end-to-end assembly of AuNR could be effectively controlled by irradiation with UV and visible light, respectively, over four cycles. By the introduction of AAP, previous disassembly limitations based on the photostationary states of azobenzenes could be solved. The combination photoresponsive interaction and selectively end-functionalized nanoparticles shows significant potential in the reversible self-assembly of inorganic-organic hybrid nanomaterials.

Independent Blue and Red Light Triggered Narcissistic Self-Sorting Self-Assembly of Colloidal Particles

The ability of living systems to self-sort different cells into separate assemblies and the ability to independently regulate different structures are one ingredient that gives rise to their spatiotemporal complexity. Here, this self-sorting behavior is replicated in a synthetic system with two types of colloidal particles; where each particle type independently self-assembles either under blue or red light into distinct clusters, known as narcissistic self-sorting. For this purpose, each particle type is functionalized either with the light-switchable protein VVDHigh or Cph1, which homodimerize under blue and red light, respectively. The response to different wavelengths of light and the high specificity of the protein interactions allows for the independent self-assembly of each particle type with blue or red light and narcissistic self-sorting. Moreover, as both of the photoswitchable protein interactions are reversible in the dark; also, the self-sorting is reversible and dynamic. Overall, the independent blue and red light controlled self-sorting in a synthetic system opens new possibilities to assemble adaptable, smart, and advanced materials similar to the complexity observed in tissues.

Light-Responsive Arylazopyrazole Gelators: From Organic to Aqueous Media and from Supramolecular to Dynamic Covalent Chemistry

Versatile photoresponsive gels based on tripodal low molecular weight gelators (LMWGs) are reported. A cyclohexane-1,3,5-tricarboxamide (CTA) core provides face-to-face hydrogen bonding and a planar conformation, inducing the self-assembly of supramolecular polymers. The CTA core was substituted with three arylazopyrazole (AAP) arms. AAP is a molecular photoswitch that isomerizes reversibly under alternating UV and green light irradiation. The E isomer of AAP is planar, favoring the self-assembly, whereas the Z isomer has a twisted structure, leading to a disassembly of the supramolecular polymers. By using tailor-made molecular design of the tripodal gelator, light-responsive organogels and hydrogels were obtained. Additionally, in the case of the hydrogels, AAP was coupled to the core through hydrazones, so that the hydrogelator and, hence, the photoresponsive hydrogel could also be assembled and disassembled by using dynamic covalent chemistry.

Intense upconversion red emission from Gd-doped NaErF4:Tm-based core/shell/shell nanocrystals under 980 and 800 nm near infrared light excitations

Strong upconversion red-emitting Gd-doped NaErF₄:Tm-based upconversion nanophosphors (UCNPs) are successfully synthesized by the formation of a core/intermediate shell/active shell structure.

Physical stimuli-responsive vesicles in drug delivery: Beyond liposomes and polymersomes

Over the past few decades, a range of vesicle-based drug delivery systems have entered clinical practice and several others are in various stages of clinical translation. While most of these vesicle constructs are lipid-based (liposomes), or polymer-based (polymersomes), recently new classes of vesicles have emerged that defy easy classification. Examples include assemblies with small molecule amphiphiles, biologically derived membranes, hybrid vesicles with two or more classes of amphiphiles, or more complex hierarchical structures such as vesicles incorporating gas bubbles or nanoparticulates in the lumen or membrane. In this review, we explore these recent advances and emerging trends at the edge and just beyond the research fields of conventional liposomes and polymersomes. A focus of this review is the distinct behaviors observed for these classes of vesicles when exposed to physical stimuli - such as ultrasound, heat, light and mechanical triggers - and we discuss the resulting potential for new types of drug delivery, with a special emphasis on current challenges and opportunities.

NIR-induced spatiotemporally controlled gene silencing by upconversion nanoparticle-based siRNA nanocarrier

Spatiotemporal control over the release or activation of biomacromolecules such as siRNA remains a significant challenge. Light-controlled release has gained popularity in recent years; however, a major limitation is that most photoactivable compounds/systems respond only to UV irradiation, but not near-infrared (NIR) light that offers a deeper tissue penetration depth and better biocompatibility. This paper reports a simple NIR-to-UV upconversion nanoparticle (UCNP)-based siRNA nanocarrier for NIR-controlled gene silencing. siRNA is complexed onto a NaYF4:Yb/Tm/Er UCNP through an azobenzene (Azo)-cyclodextrin (CD) host-guest interaction. The UV emission generated by the NIR-activated UCNP effectively triggers the trans-to-cis photoisomerization of azobenzene, thus leading to the release of siRNA due to unmatched host-guest pairs. The UCNP-siRNA complexes are also functionalized with PEG (i.e., UCNP-(CD/Azo)-siRNA/PEG NPs), targeting ligands (i.e., EGFR-specific GE11 peptide), acid-activatable cell-penetrating peptides (i.e., TH peptide), and imaging probes (i.e., Cy5 fluorophore). The UCNP-(CD/Azo)-siRNA/PEG NPs with both GE11 and TH peptides display a high level of cellular uptake and an excellent endosomal/lysosomal escape capability. More importantly, NIR-controlled spatiotemporal knockdown of GFP expression is successfully achieved in both a 2D monolayer cell model and a 3D multicellular tumor spheroid model. Thus, this simple and versatile nanoplatform has great potential for the selective activation or release of various biomacromolecules.

Lanthanide-Doped Upconversion Nanocarriers for Drug and Gene Delivery

Compared to traditional cancer treatments, drug/gene delivery is an advanced, safe, and efficient method. Nanoparticles are widely used as nanocarriers in a drug/gene delivery system due to their long circulation time and low multi-drug resistance. In particular, lanthanide-doped upconversion nanoparticles (UCNPs) that can emit UV and visible light by near-infrared (NIR) upconversion demonstrated more efficient and safer drug/gene delivery. Because of the low penetration depth of UV and visible light, a photoinduced reaction such as photocleavage or photoisomerization has proven restrictive. However, NIR light has high tissue penetration depth and stimulates the photoinduced reaction through UV and visible emissions from lanthanide-doped UCNPs. This review discusses the optical properties of UCNPs that are useful in bioapplications and drug/gene delivery systems using the UCNPs as a photoreaction inducer.

Arylazopyrazole Photoswitches in Aqueous Solution: Substituent Effects, Photophysical Properties, and Host-Guest Chemistry

Getting the green light! Substituted arylazopyrazoles (AAPs) have been investigated as supramolecular photoswitches in aqueous solution. Selective photostationary states (PSSs) and improved binding affinities to β-cyclodextrin have been determined. The experimental findings are supported by results from DFT calculations.

A Modular System for the Design of Stimuli-Responsive Multifunctional Nanoparticle Aggregates by Use of Host-Guest Chemistry

A self-assembly approach for the design of multifunctional nanomaterials consisting of different nanoparticles (gold, iron oxide, and lanthanide-doped LiYF₄ ) is developed. This modular system takes advantage of the light-responsive supramolecular host-guest chemistry of β-cyclodextrin and arylazopyrazole, which enables the dynamic and reversible self-assembly of particles to spherical nanoparticle aggregates in aqueous solution. Due to the magnetic iron oxide nanoparticles, the aggregates can be manipulated by an external magnetic field leading to the formation of linear structures. As a result of the integration of upconversion nanoparticles, the aggregates are additionally responsive to near-infrared light and can be redispersed by use of the upconversion effect. By varying the nanoparticle and linker concentrations the composition, size, shape, and properties of the multifunctional nanoparticle aggregates can be fine-tuned.

Near-Infrared Photoswitching of Azobenzenes under Physiological Conditions

Biological tissue exhibits an absorbance minimum in the near-infrared between 700 and 900 nm that permits deep penetration of light. Molecules that undergo photoisomerization in this bio-optical window are highly desirable as core structures for the development of photopharmaceuticals and as components of chemical-biological tools. We report the systematic design, synthesis, and testing of an azobenzene derivative tailored to undergo single-photon photoswitching with near-infrared light under physiological conditions. A fused dioxane ring and a methoxy substituent were used to place oxygen atoms in all four ortho positions, as well as two meta positions, relative to the azobenzene N═N double bond. This substitution pattern, together with a para pyrrolidine group, raises the pK_a of the molecule so that it is protonated at physiological pH and absorbs at wavelengths >700 nm. This azobenzene derivative, termed DOM-azo, is stable for months in neutral aqueous solutions, undergoes trans-to-cis photoswitching with 720 nm light, and thermally reverts to the stable trans isomer with a half-life near 1 s.

Stabilization of High Oxidation State Upconversion Nanoparticles by N-Heterocyclic Carbenes

The stabilization of high oxidation state nanoparticles by N-heterocyclic carbenes is reported. Such nanoparticles represent an important subset in the field of nanoparticles, with different and more challenging requirements for suitable ligands compared to elemental metal nanoparticles. N-Heterocyclic carbene coated NaYF₄ :Yb,Tm upconversion nanoparticles were synthesized by a ligand-exchange reaction from a well-defined precursor. This new photoactive material was characterized in detail and employed in the activation of photoresponsive molecules by low-intensity near-infrared light (λ=980 nm).

Hierarchical supramolecular hydrogels: self-assembly by peptides and photo-controlled release via host–guest interaction

Using photoresponsive host–guest chemistry, three different payloads can be photo-released successively from the same peptide hydrogel.