Stimuli-Responsive DNA-Based Hydrogels on Surfaces for Switchable Bioelectrocatalysis and Controlled Release of Loads.

The assembly of enzyme [glucose oxidase (GOx)]-loaded stimuli-responsive DNA-based hydrogels on electrode surfaces, and the triggered control over the stiffness of the hydrogels, provides a means to switch the bioelectrocatalytic functions of the hydrogels. One system includes the assembly of GOx-loaded, pH-responsive, hydrogel matrices cross-linked by two cooperative nucleic acid motives comprising permanent duplex nucleic acids and "caged" i-motif pH-responsive duplexes. Bioelectrocatalyzed oxidation of glucose leads to the formation of gluconic acid that acidifies the hydrogel resulting in the separation of the i-motif constituents and lowering the hydrogel stiffness. Loading of the hydrogel matrices with insulin results in the potential-triggered, glucose concentration-controlled, switchable release of insulin from the hydrogel-modified electrodes. The switchable bioelectrocatalyzed release of insulin is demonstrated in the presence of ferrocenemethanol as a diffusional electron mediator or by applying an electrically wired integrated matrix that includes ferrocenyl-modified GOx embedded in the hydrogel. The second GOx-loaded, stimuli-responsive, DNA-based hydrogel matrix associated with the electrode includes a polyacrylamide hydrogel cooperatively cross-linked by duplex nucleic acids and "caged" G-quadruplex-responsive duplexes. The hydrogel matrix undergoes K+-ions/crown ether-triggered stiffness changes by the cyclic K+-ion-stimulated formation of G-quadruplexes (lower stiffness) and the crown ether-induced separation of the G-quadruplexes (higher stiffness). The hydrogel matrices demonstrate switchable bioelectrocatalytic functions guided by the stiffness properties of the hydrogels.

Aptamer-Functionalized Ce4+-Ion-Modified C-Dots: Peroxidase Mimicking Aptananozymes for the Oxidation of Dopamine and Cytotoxic Effects toward Cancer Cells.

Aptamer-functionalized Ce4+-ion-modified C-dots act as catalytic hybrid systems, aptananozymes, catalyzing the H2O2 oxidation of dopamine. A series of aptananozymes functionalized with different configurations of the dopamine binding aptamer, DBA, are introduced. All aptananozymes reveal substantially enhanced catalytic activities as compared to the separated Ce4+-ion-modified C-dots and aptamer constituents, and structure-catalytic functions between the structure and binding modes of the aptamers linked to the C-dots are demonstrated. The enhanced catalytic functions of the aptananozymes are attributed to the aptamer-induced concentration of the reaction substrates in spatial proximity to the Ce4+-ion-modified C-dots catalytic sites. The oxidation processes driven by the Ce4+-ion-modified C-dots involve the formation of reactive oxygen species (•OH radicals). Accordingly, Ce4+-ion-modified C-dots with the AS1411 aptamer or MUC1 aptamer, recognizing specific biomarkers associated with cancer cells, are employed as targeted catalytic agents for chemodynamic treatment of cancer cells. Treatment of MDA-MB-231 breast cancer cells and MCF-10A epithelial breast cells, as control, with the AS1411 aptamer- or MUC1 aptamer-modified Ce4+-ion-modified C-dots reveals selective cytotoxicity toward the cancer cells. In vivo experiments reveal that the aptamer-functionalized nanoparticles inhibit MDA-MB-231 tumor growth.

Aptamer-Protein Structures Guide In Silico and Experimental Discovery of Aptamer-Short Peptide Recognition Complexes or Aptamer-Amino Acid Cluster Complexes.

A method to computationally and experimentally identify aptamers against short peptides or amino acid clusters is introduced. The method involves the selection of a well-defined protein aptamer complex and the extraction of the peptide sequence participating in the binding of the protein to the aptamer. The subsequent fragmentation of the peptide sequence into short peptides and the in silico docking-guided identification of affinity complexes between the miniaturized peptides and the antiprotein aptamer, followed by experimental validation of the binding features of the short peptides with the antiprotein aptamers, leads to the identification of new short peptide-aptamer complexes. This is exemplified with the identification of the pentapeptide RYERN as the scaffold that binds thrombin to the DNA thrombin aptamer (DNA TA). In silico docking studies followed by microscale thermophoresis (MST) experiments demonstrate that the miniaturized tripeptides RYE, YER, and ERN reveal selective binding affinities toward the DNA TA. In addition, docking and MST experiments show that the ribonucleotide-translated RNA TA shows related binding affinities of YER to the DNA TA. Most importantly, we demonstrate that the separated amino acids Y/E/R assemble as a three amino acid cluster on the DNA TA and RNA TA aptamers in spatial configurations similar to the tripeptide YER on the respective aptamers. The clustering phenomenon is selective for the YER tripeptide system. The method to identify binding affinities of miniaturized peptides to known antiprotein aptamers and the specific clustering of single amino acids on the aptamers is further demonstrated by in silico and experimental identification of the binding of the tripeptide RET and the selective clustering of the separated amino acids R/E/T onto a derivative of the AS1411 aptamer against the nucleolin receptor protein.

Aptamer-Modified Au Nanoparticles: Functional Nanozyme Bioreactors for Cascaded Catalysis and Catalysts for Chemodynamic Treatment of Cancer Cells.

Polyadenine-stabilized Au nanoparticles (pA-AuNPs) reveal dual nanozyme catalytic activities toward the H2O2-mediated oxidation of dopamine to aminochrome and toward the aerobic oxidation of glucose to gluconic acid and H2O2. The conjugation of a dopamine-binding aptamer (DBA) to the pA-AuNPs yields aptananozyme structures catalyzing simultaneously the H2O2-mediated oxidation of dopamine to aminochrome through the aerobic oxidation of glucose. A set of aptananozymes consisting of DBA conjugated through the 5'- or 3'-end directly or spacer bridges to pA-AuNPs were synthesized. The set of aptananozymes revealed enhanced catalytic activities toward the H2O2-catalyzed oxidation of dopamine to dopachrome, as compared to the separated pA-AuNPs and DBA constituents, and structure-function relationships within the series of aptananozymes were demonstrated. The enhanced catalytic function of the aptananozymes was attributed to the concentration of the dopamine at the catalytic interfaces by means of aptamer-dopamine complexes. The dual catalytic activities of aptananozymes were further applied to design bioreactors catalyzing the effective aerobic oxidation of dopamine in the presence of glucose. Mechanistic studies demonstrated that the aptananozymes generate reactive oxygen species. Accordingly, the AS1411 aptamer, recognizing the nucleolin receptor associated with cancer cells, was conjugated to the pA-AuNPs, yielding a nanozyme for the chemodynamic treatment of cancer cells. The AS1411 aptamer targets the aptananozyme to the cancer cells and facilitates the selective permeation of the nanozyme into the cells. Selective cytotoxicity toward MDA-MB-231 breast cancer cells (ca. 70% cell death) as compared to MCF-10A epithelial cells (ca. 2% cell death) is demonstrated.

miRNA-Guided Imaging and Photodynamic Therapy Treatment of Cancer Cells Using Zn(II)-Protoporphyrin IX-Loaded Metal-Organic Framework Nanoparticles.

An analytical platform for the selective miRNA-21-guided imaging of breast cancer cells and miRNA-221-guided imaging of ovarian cancer cells and the selective photodynamic therapy (PDT) of these cancer cells is introduced. The method is based on Zn(II)-protoporphyrin IX, Zn(II)-PPIX-loaded UiO-66 metal-organic framework nanoparticles, NMOFs, gated by two hairpins Hi/Hj through ligation of their phosphate residues to the vacant Zr4+-ions associated with the NMOFs. The hairpins are engineered to include the miRNA recognition sequence in the stem domain of Hi, and in the Hi and Hj, partial locked stem regions of G-quadruplex subunits. Intracellular phosphate-ions displace the hairpins, resulting in the release of the Zn(II)-PPIX and intracellular miRNAs open Hi, and this triggers the autonomous cross-opening of Hi and Hj. This activates the interhairpin hybridization chain reaction and leads to the assembly of highly fluorescent Zn(II)-PPIX-loaded G-quadruplex chains. The miRNA-guided fluorescent chains allow selective imaging of cancer cells. Moreover, PDT with visible light selectively kills cancer cells and tumor cells through the formation of toxic reactive oxygen species.

Aptamer-Modified Cu2+-Functionalized C-Dots: Versatile Means to Improve Nanozyme Activities-"Aptananozymes".

The covalent linkage of aptamer binding sites to nanoparticle nanozymes is introduced as a versatile method to improve the catalytic activity of nanozymes by concentrating the reaction substrates at the catalytic nanozyme core, thereby emulating the binding and catalytic active-site functions of native enzymes. The concept is exemplified with the synthesis of Cu2+ ion-functionalized carbon dots (C-dots), modified with the dopamine binding aptamer (DBA) or the tyrosinamide binding aptamer (TBA), for the catalyzed oxidation of dopamine to aminochrome by H2O2 or the oxygenation of l-tyrosinamide to the catechol product, which is subsequently oxidized to amidodopachrome, in the presence of H2O2/ascorbate mixture. Sets of structurally functionalized DBA-modified Cu2+ ion-functionalized C-dots or sets of structurally functionalized TBA-modified Cu2+ ion-functionalized C-dots are introduced as nanozymes of superior catalytic activities (aptananozymes) toward the oxidation of dopamine or the oxygenation of l-tyrosinamide, respectively. The aptananozymes reveal enhanced catalytic activities as compared to the separated catalyst and respective aptamer constituents. The catalytic functions of the aptananozymes are controlled by the structure of the aptamer units linked to the Cu2+ ion-functionalized C-dots. In addition, the aptananozyme shows chiroselective catalytic functions demonstrated by the chiroselective-catalyzed oxidation of l/d-DOPA to l/d-dopachrome. Binding studies of the substrates to the different aptananozymes and mechanistic studies associated with the catalytic transformations are discussed.

Thermoplasmonic-Triggered Release of Loads from DNA-Modified Hydrogel Microcapsules Functionalized with Au Nanoparticles or Au Nanorods.

Microcapsules consisting of hydrogel shells cross-linked by glucosamine-boronate ester complexes and duplex nucleic acids, loaded with dyes or drugs and functionalized with Au nanoparticles (Au NPs) or Au nanorods (Au NRs), are developed. Irradiation of Au NPs or Au NRs results in the thermoplasmonic heating of the microcapsules, and the dissociation of the nucleic acid cross-linkers. The separation of duplex nucleic acid cross-linkers leads to low-stiffness hydrogel shells, allowing the release of loads. Switching off the light-induced plasmonic heating results in the regeneration of stiff hydrogel shells protecting the microcapsules, leading to the blockage of release processes. The thermoplasmonic release of tetramethylrhodamine-dextran, Texas Red-dextran, doxorubicin-dextran (DOX-D), or camptothecin-carboxymethylcellulose (CPT-CMC) from the microcapsules is introduced. By loading the microcapsules with two different drugs (DOX-D and CPT-CMC), the light-controlled dose release is demonstrated. Cellular experiments show efficient permeation of Au NPs/DOX-D or Au NRs/DOX-D microcapsules into MDA-MB-231 cancer cells and inefficient uptake by MCF-10A epithelial breast cells. Cytotoxicity experiments reveal selective thermoplasmon-induced cytotoxicity of the microcapsules toward MDA-MB-231 cancer cells as compared to MCF-10A cells. Also, selective cytotoxicity towards MDA-MB-231 cancer cells upon irradiation of the Au NPs- and Au NRs-functionalized microcapsules at λ = 532 or 910 nm is demonstrated.

Triggered Release of Loads from Microcapsule-in-Microcapsule Hydrogel Microcarriers: En-Route to an "Artificial Pancreas".

A method to assemble stimuli-responsive nucleic acid-based hydrogel-stabilized microcapsule-in-microcapsule systems is introduced. An inner aqueous compartment stabilized by a stimuli-responsive hydrogel-layer (∼150 nm) provides the inner microcapsule (diameter ∼2.5 µm). The inner microcapsule is separated from an outer aqueous compartment stabilized by an outer stimuli-responsive hydrogel layer (thickness of ∼150 nm) that yields the microcapsule-in-microcapsule system. Different loads, e.g., tetramethyl rhodamine-dextran (TMR-D) and CdSe/ZnS quantum dots (QDs), are loaded in the inner and outer aqueous compartments. The hydrogel layers exist in a higher stiffness state that prevents inter-reservoir or leakage of the loads from the respective aqueous compartments. Subjecting the inner hydrogel layer to Zn2+-ions and/or the outer hydrogel layer to acidic pH or crown ether leads to the triggered separation of the bridging units associated with the respective hydrogel layers. This results in the hydrogel layers of lower stiffness allowing either the mixing of the loads occupying the two aqueous compartments, the guided release of the load from the outer aqueous compartment, or the release of the loads from the two aqueous compartments. In addition, a pH-responsive microcapsule-in-microcapsule system is loaded with glucose oxidase (GOx) in the inner aqueous compartment and insulin in the outer aqueous compartment. Glucose permeates across the two hydrogel layers resulting in the GOx catalyzed aerobic oxidation of glucose to gluconic acid. The acidification of the microcapsule-in-microcapsule system leads to the triggered unlocking of the outer, pH-responsive hydrogel layer and to the release of insulin. The pH-stimulated release of insulin is controlled by the concentration of glucose. While at normal glucose levels, the release of insulin is practically prohibited, the dose-controlled release of insulin in the entire diabetic range  is demonstrated. Also, switchable ON/OFF release of insulin is achieved highlighting an autonomous glucose-responsive microdevice operating as an "artificial pancreas" for the release of insulin.

Redox-Switchable Binding Properties of the ATP-Aptamer.

In this study, we report on a redox-controllable and reversible complete "ON"/"OFF"-switchable aptamer binding to ATP. A series of methylene blue-modified ATP-aptamers was synthesized, revealing improved binding affinities toward ATP as compared to the nonmodified aptamer. These binding affinities were dependent on the conjugation site of the redox label on the aptamer scaffold. Importantly, we find that the oxidized methylene blue-modified aptamers bind to ATP with micromolar affinity, while the reduced form lacks binding affinity toward ATP, resulting in an unprecedented complete "ON"/"OFF" redox-controllable aptamer switch. We demonstrate the cyclic "ON"/"OFF" binding of ATP to the methylene blue-functionalized aptamer through cyclic oxidation and reduction of the redox label using both chemical and electrochemical means. Molecular dynamics and docking simulations were performed to account for the redox-switchable properties of the conjugated aptamers and to rationalize the enhanced binding affinities of the different aptamer designs.

Molecularly Imprinted Sites Translate into Macroscopic Shape-Memory Properties of Hydrogels.

The polymerization of acrylamide, dopamine methacrylamide, and bis-acrylamide in the presence of one of the electron acceptors, N,N'-dimethyl-4,4'-bipyridinium, (1), N,N'-dimethylbipyridinium-4,4'-ethylene, (2), or bipyridinium dithienylethene, (3), yields hydrogel matrices of high stiffness that are cooperatively cross-linked by bis-acrylamide and electron donor (dopamine)-acceptor complexes. Washing off the diffusional electron acceptor units yields molecularly imprinted matrices of lower stiffness, stabilized only by the bis-acrylamide bridges that include specific binding sites for the selective association of the electron acceptor (1), (2), or (3). These imprinted hydrogel matrices show selective recovery of the stiff properties upon binding the respective electron acceptor units to the imprinted sites. The control over the stiffness properties enables the development of shape-memory, molecularly imprinted hydrogels and stiffness-based sensors. The results show how molecularly imprinted sites translate into macroscopic shape-memory properties of hydrogels.

Artificial Photosynthesis with Electron Acceptor/Photosensitizer-Aptamer Conjugates.

Sequence-specific aptamers act as functional scaffolds for the assembly of photosynthetic model systems. The Ru(II)-tris-bipyridine photosensitizer is conjugated by different binding modes to the antityrosinamide aptamer to yield a set of photosensitizer-aptamer binding scaffolds. The N-methyl-N'-(3-aminopropane)-4,4'-bipyridinium electron acceptor, MV2+, is covalently linked to tyrosinamide, TA, to yield the conjugate TA-MV2+. The tyrosinamide unit in TA-MV2+ acts as a ligand for anchoring TA-MV2+ to the Ru(II)-tris-bipyridine-aptamer scaffold, generating the diversity of photosensitizer-aptamer/electron acceptor supramolecular conjugates. Effective electron transfer quenching in the photosynthetic model systems is demonstrated, and the quenching efficiencies are controlled by the structural features of the conjugates. The redox species generated by the photosensitizer-aptamer/electron acceptor supramolecular systems mediate the ferredoxin-NADP+ reductase, FNR, catalyzed synthesis of NADPH, and the Pt-nanoparticle-catalyzed evolution of hydrogen (H2). The novelty of the study rests on the unprecedented use of aptamer scaffolds as functional units for organizing photosynthetic model systems.

DNA-Based Hydrogels Loaded with Au Nanoparticles or Au Nanorods: Thermoresponsive Plasmonic Matrices for Shape-Memory, Self-Healing, Controlled Release, and Mechanical Applications.

Gold nanoparticles (AuNPs) or gold nanorods (AuNRs) are loaded in polyacrylamide hydrogels cooperatively cross-linked by bis-acrylamide and nucleic acid duplexes or boronate ester-glucosamine and nucleic acid duplexes. The thermoplasmonic properties of AuNPs and AuNRs are used to control the stiffness of the hydrogels. The irradiation of the AuNP-loaded (λ = 532 nm) or the AuNR-loaded (λ = 808 nm) hydrogels leads to thermoplasmonic heating of the hydrogels, the dehybridization of the DNA duplexes, and the formation of hydrogels with lower stiffness. By ON/OFF irradiation, the hydrogels are switched between low- and high-stiffness states. The reversible control over the stiffness properties of the hydrogels is used to develop shape-memory hydrogels and self-healing soft materials and to tailor thermoplasmonic switchable drug release. In addition, by designing bilayer composites of AuNP- and AuNR-loaded hydrogels, a reversible thermoplasmonic, light-induced bending is demonstrated, where the bending direction is controlled by the stress generated in the respective bilayer composite.

Chemical and photochemical DNA "gears" reversibly control stiffness, shape-memory, self-healing and controlled release properties of polyacrylamide hydrogels.

A new class of stimuli-responsive DNA-based polyacrylamide hydrogels is described. They consist of glucosamine-boronate ester-crosslinked polyacrylamide chains being cooperatively bridged by stimuli-responsive nucleic acids. The triggered closure and dissociation of the stimuli-responsive units lead to switchable stiffness properties of the hydrogel. One hydrogel includes glucosamine-boronate esters and K+-ion-stabilized G-quadruplex units as cooperative crosslinkers. The hydrogel bridged by the two motifs reveals high stiffness, whereas the separation of the G-quadruplex bridges by 18-crown-6-ether yields a low stiffness hydrogel. By cyclic treatment of the hydrogel with K+-ions and 18-crown-6-ether, it is reversibly cycled between high and low stiffness states. The second system involves a photo-responsive hydrogel that reveals light-induced switchable stiffness functions. The polyacrylamide chains are cooperatively crosslinked by glucosamine-boronate esters and duplex nucleic acid bridges stabilized by trans-azobenzene intercalator units. The resulting hydrogel reveals high stiffness. Photoisomerization of the trans-azobenzene units to the cis-azobenzene states results in the separation of the duplex nucleic acid bridges and the formation of a low stiffness hydrogel. The control over the stiffness properties of the hydrogel matrices by means of K+-ions/crown ether or photoisomerizable trans-azobenzene/cis-azobenzene units is used to develop shape-memory, self-healing, and controlled drug-release hydrogel materials.

Multi-triggered Supramolecular DNA/Bipyridinium Dithienylethene Hydrogels Driven by Light, Redox, and Chemical Stimuli for Shape-Memory and Self-Healing Applications.

Multi-triggered DNA/bipyridinium dithienylethene (DTE) hybrid carboxymethyl cellulose (CMC)-based hydrogels are introduced. DTE exhibits cyclic and reversible photoisomerization properties, switching between the closed state (DTEc), the electron acceptor, and the open isomer (DTEo) that lacks electron acceptor properties. One system introduces a dual stimuli-responsive hydrogel containing CMC chains modified with electron donor dopamine sites and self-complementary nucleic acids. In the presence of DTEc and the CMC scaffold, a stiff hydrogel is formed, cooperatively stabilized by dopamine/DTEc donor-acceptor interactions and by duplex nucleic acids. The cyclic and reversible formation and dissociation of the supramolecular donor-acceptor interactions, through light-induced photoisomerization of DTE, or via oxidation and subsequent reduction of the dopamine sites, leads to hydrogels of switchable stiffness. Another system introduces a stimuli-responsive hydrogel triggered by one of three alternative signals. The stiff, multi-triggered hydrogel consists of CMC chains cross-linked by dopamine/DTEc donor-acceptor interactions, and by supramolecular K+-stabilized G-quadruplexes. The G-quadruplexes are reversibly separated in the presence of 18-crown-6 ether and reformed upon the addition of K+. The stiff hydrogel undergoes reversible transitions between high-stiffness and low-stiffness states triggered by light, redox agents, or K+/crown ether. The hybrid donor-acceptor/G-quadruplex cross-linked hydrogel shows shape-memory and self-healing features. By using three different triggers and two alternative memory-codes, e.g., the dopamine/DTEc or the K+-stabilized G-quadruplexes, the guided shape-memory function of the hydrogel matrices is demonstrated.

Shape-memory and self-healing functions of DNA-based carboxymethyl cellulose hydrogels driven by chemical or light triggers.

Photoresponsive nucleic acid-based carboxymethyl cellulose (CMC) hydrogels are synthesized, and their application as shape-memory and self-healing functional matrices are discussed. One system involves the preparation of a carboxymethyl cellulose hydrogel crosslinked by self-complementary nucleic acid duplexes and by photoresponsive trans-azobenzene/ß-cyclodextrin (ß-CD) supramolecular complexes. Photoisomerization of the trans-azobenzene to the cis-azobenzene results in a hydrogel exhibiting lower stiffness due to the separation of the azobenzene/ß-CD bridging units. The hydrogel is switched between high and low stiffness states by the cyclic and reversible light-induced isomerization of the azobenzene units between the trans and cis states. The light-controlled stiffness properties of the hydrogel are used to develop a shape-memory hydrogel, where the duplex bridging units act as permanent memory in the quasi-liquid shapeless state of the hydrogel. A second system in the study is a carboxymethyl cellulose hydrogel crosslinked by the K+-stabilized G-quadruplex bridging units and by trans-azobenzene/ß-CD complexes. The resulting hydrogel includes dual-trigger functionalities, where the trans-azobenzene/ß-CD complexes can be reversibly formed and dissociated through the trans and cis photoisomerization of the azobenzene units, and the K+-stabilized G-quadruplexes can be reversibly dissociated and reformed in the presence of 18-crown-6-ether/K+-ions. The signal-responsive crosslinked hydrogel reveals controlled stiffness properties, where the hydrogel crosslinked by the trans-azobenzene/ß-CD and K+-ion-stabilized G-quadruplex reveals high stiffness and the hydrogel crosslinked only by the K+-ion-stabilized G-quadruplexes or only by the trans-azobenzene/ß-CD complexes reveals low stiffness properties. The controlled stiffness properties of the hydrogel are used to develop shape-memory hydrogels, where the trans-azobenzene/ß-CD complexes or the K+-ion-stabilized G-quadruplexes act as permanent memories in the shapeless and quasi-liquid states of the hydrogels. In addition, the hydrogel that includes two types of stimuli-responsive crosslinking units is used as a self-healing matrix, where each of the triggers guides the self-healing processes.

Catalyzed and Electrocatalyzed Oxidation of l-Tyrosine and l-Phenylalanine to Dopachrome by Nanozymes.

Catalyzed oxygen insertion into C-H bonds represents a continuous challenge in chemistry. Particularly, driving this process at ambient temperature and aqueous media represents a "holy grail" in catalysis. We report on the catalyzed cascade transformations of l-tyrosine or l-phenylalanine to dopachrome in the presence of l-ascorbic acid/H2O2 as oxidizing mixture and CuFe-Prussian Blue-like nanoparticles, Fe3O4 nanoparticles or Au nanoparticles as catalysts. The process involves the primary transformation of l-tyrosine to l-DOPA that is further oxidized to dopachrome. The transformation of l-phenylalanine to dopachrome in the presence of CuFe-Prussian Blue-like nanoparticles and l-ascorbic acid/H2O2 involves in the first step the formation of l-tyrosine and, subsequently, the operation of the catalytic oxidation cascade of l-tyrosine to l-DOPA and dopachrome. Electron spin resonance experiments demonstrate that ascorbate radicals and hydroxyl radicals play cooperative functions in driving the different oxygen-insertion processes. In addition, the aerobic elecrocatalyzed oxidation of l-tyrosine to dopachrome in the presence of naphthoquinone-modified Fe3O4 nanoparticles and l-ascorbic acid is demonstrated. In this system, magnetic-field attraction of the naphthoquinone-modified Fe3O4 nanoparticles onto the electrode allows the quinone-mediated electrocatalyzed reduction of O2 to H2O2 (bias potential -0.5 V vs SCE). The electrogenerated H2O2 is then utilized to promote the transformation of l-tyrosine to dopachrome in the presence of l-ascorbic acid and Fe3O4 catalyst.

Targeted VEGF-triggered release of an anti-cancer drug from aptamer-functionalized metal-organic framework nanoparticles.

Amino-triphenyl dicarboxylate-bridged Zr4+ metal-organic framework nanoparticles (NMOFs), 100-130 nm, are modified with a nucleic acid complementary to the VEGF aptamer. The nucleic acid-functionalized NMOFs were loaded with the anti-cancer drug doxorubicin (or Rhodamine 6G as a drug model), and the loaded NMOFs were capped by hybridization with the VEGF aptamer that yielded VEGF-responsive duplex nucleic acid gates. In the presence of VEGF, a biomarker over-expressed in cancer cells, selective unlocking of the gates proceeds through the formation of VEGF/aptamer complexes, resulting in the release of the loads. In addition, the VEGF aptamer locking units were conjugated to the AS1411 aptamer sequence that binds to nucleolin receptors associated with cancer cells, resulting in the construction of cancer-cell targeted VEGF-responsive doxorubicin-loaded NMOFs. The different drug-loaded stimuli-responsive NMOFs reveal selective permeation into MDA-MB-231 breast cancer cells, compared to their incorporation into normal MCF-10A breast cells, with a two-fold enhanced incorporation into the MDA-MB-231 cells of the AS1411 aptamer-functionalized NMOFs. Cytotoxicity experiments revealed impressive selective apoptosis of the doxorubicin-loaded NMOFs towards the MDA-MB-231 cancer cells compared to the normal MCF-10A breast cells. A 55% and 70% MDA-MB-231 cell apoptosis was observed upon subjecting the cells to the VEGF aptamer and the VEGF aptamer/AS1411 aptamer conjugate-caged NMOFs, respectively, for a time-interval of three days, where only <10% apoptosis of the MCF-10A cells was observed under similar conditions.

Reversible Modulation of DNA-Based Hydrogel Shapes by Internal Stress Interactions.

We present the assembly of asymmetric two-layer hybrid DNA-based hydrogels revealing stimuli-triggered reversibly modulated shape transitions. Asymmetric, linear hydrogels that include layer-selective switchable stimuli-responsive elements that control the hydrogel stiffness are designed. Trigger-induced stress in one of the layers results in the bending of the linear hybrid structure, thereby minimizing the elastic free energy of the systems. The removal of the stress by a counter-trigger restores the original linear bilayer hydrogel. The stiffness of the DNA hydrogel layers is controlled by thermal, pH (i-motif), K+ ion/crown ether (G-quadruplexes), chemical (pH-doped polyaniline), or biocatalytic (glucose oxidase/urease) triggers. A theoretical model relating the experimental bending radius of curvatures of the hydrogels with the Young's moduli and geometrical parameters of the hydrogels is provided. Promising applications of shape-regulated stimuli-responsive asymmetric hydrogels include their use as valves, actuators, sensors, and drug delivery devices.

Switchable bifunctional stimuli-triggered poly-N-isopropylacrylamide/DNA hydrogels.

DNA-tethered poly-N-isopropylacrylamide copolymer chains, pNIPAM, that include nucleic acid tethers have been synthesized. They are capable of inducing pH-stimulated crosslinking of the chains by i-motif structures or to be bridged by Ag(+) ions to form duplexes. The solutions of pNIPAM chains undergo crosslinking at pH 5.2 or in the presence of Ag(+) ions to form hydrogels. The hydrogels reveal switchable hydrogel-to-solution transitions by the reversible crosslinking of the chains at pH 5.2 and the separation of the crosslinking units at pH 7.5, or by the Ag(+) ion-stimulated crosslinking of the chains and the reverse dissolution of the hydrogel by the cysteamine-induced elimination of the Ag(+) ions. The DNA-crosslinked hydrogels are thermosensitive and undergo reversible temperature-controlled hydrogel-to-solid transitions. The solid pNIPAM matrices are protected against the OH(-) or cysteamine-stimulated dissociation to the respective polymer solutions.