Controlled-access hollow mechanized silica nanocontainers.

A new category of mechanized nanoparticles, consisting of a hollow mesoporous silica spherical framework controlled by a supramolecular system containing the alpha-cyclodextrin (alpha-CD) ring on a stalk that is tethered to the pore openings on the nanosphere, is synthesized and tested. Construction of the nanovalve relies on the hydrogen-bonding interaction between alpha-CD and the stalk. The stalk is bonded to the nanoparticle chemically and contains an anilino group that is located on the end of the linker molecule that is closest to the pore entrance. When the alpha-CD ring is complexed with the stalk at neutral pH, the bulky cyclic component is located near the pore openings, thereby blocking departure of cargo molecules that were loaded in the nanopores and hollow interior of the particle. Protonation of the nitrogen atoms at lower pH causes the binding affinity to decrease, releasing the alpha-CD and allowing the cargo molecules to escape. The properties of this newly designed pH-responsive nanovalve are compared to those of conventional mesoporous silica nanoparticles. The on-command pH-activated release is measured using luminescence spectroscopy. The effect of different stalk lengths and pH conditions on the release of fluorescent dye cargo molecules is measured.

Light-operated mechanized nanoparticles.

Mesoporous silica (MCM-41) nanoparticles modified by azobenzene derivatives, capable of storing small molecules and releasing them following light irradiation, have been fabricated and characterized. In the presence of the beta-cyclodextrin and/or pyrene-modified beta-cyclodextrin rings, the beta-cyclodextrin and/or pyrene-modified beta-cyclodextrin rings will thread onto the azobenzene-containing stalks and bind to trans-azobenzene units to form the pseudorotaxanes, thus sealing the nanopores and stopping release of the cargo. Upon irradiation, the isomerization of trans-to-cis azobenzene units leads to the dissociation of the beta-cyclodextrin and/or pyrene-modified beta-cyclodextrin rings from the stalks, thus opening the gates to the nanopores and releasing the cargo.

A light-gated STOP-GO molecular shuttle.

Degenerate [2]rotaxanes, with their two identical binding sites, generally exhibit equilibrium dynamics with free energies of activation (DeltaG(double dagger)) for the shuttling process starting as low as 10 kcal x mol(-1). This DeltaG(double dagger) value can be raised quite dramatically by inserting "speed bumps" in the form of steric and/or electrostatic barriers into the linkers between the two identical binding sites. In our more recent research targeted toward the exploitation of the 4,4'-azobiphenyloxy unit (ABP) as a light-operated gate, we decided to introduce (i) four methyl groups on the one hand and (ii) four fluorine atoms on the other, at the 3,5,3',5'-positions of the ABP units to curtail binding by the CBPQT(4+) ring if not sterically in the case of i, then electronically in the case of ii. The first approach led to a gate (ABP-Me(4)) that remains closed all the time, whereas the second approach affords a gate (ABP-F(4)) that we can close with UV light and open with visible light. Herein, we show how light can be used, in conjunction with thermal energy, to raise and lower the free energy barrier at will and, in so doing, impart STOP and GO instructions upon the operation of a molecular shuttle.

pH clock-operated mechanized nanoparticles.

Mechanized nanoparticles (MNPs) consisting of supramolecular machines attached to the surface of mesoporous silica nanoparticles are designed to release encapsulated guest molecules controllably under pH activation. The molecular machines are comprised of cucurbit[6]uril (CB[6]) rings that encircle tethered trisammonium stalks and can be tuned to respond under specific pH conditions through chemical modification of the stalks. Luminescence spectroscopy demonstrates that the MNPs are able to contain guest molecules within nanopores at neutral pH levels and then release them once the pH is lowered or raised.

Snap-top nanocarriers.

An approach to the design and fabrication of mechanized mesoporous silica nanoparticles is demonstrated at the proof of principle level. It relies on the reductive cleavage of disulfide bonds within an integrated nanosystem, wherein surface-bound rotaxanes incorporate disulfide bonds in their stalks, which are encircled by cucurbit[6]uril or alpha-cyclodextrin rings, until reductive chemistry is performed, resulting in the snapping of the stalks of the rotaxanes, leading to cargo release from the inside of the nanoparticles.

A tristable [2]pseudo[2]rotaxane.

A strategy towards increasing the lifetime of the metastable state of a [2]rotaxane incorporating tetrathiafulvalene, 1,5-dioxynaphthalene and bipyridinium (BIPY(2+)) is presented. Incorporation of BIPY(2+) served multiple roles as an electrostatic barrier to relaxation, a supramolecular recognition site for bis-1,5-dioxynaphthalene[38]crown-10 macrocycle, and upon reduction a recognition site for the mechanically bonded cyclobis(paraquat-p-phenylene) ring.

Radically enhanced molecular recognition.

The tendency for viologen radical cations to dimerize has been harnessed to establish a recognition motif based on their ability to form extremely strong inclusion complexes with cyclobis(paraquat-p-phenylene) in its diradical dicationic redox state. This previously unreported complex involving three bipyridinium cation radicals increases the versatility of host-guest chemistry, extending its practice beyond the traditional reliance on neutral and charged guests and hosts. In particular, transporting the concept of radical dimerization into the field of mechanically interlocked molecules introduces a higher level of control within molecular switches and machines. Herein, we report that bistable and tristable [2]rotaxanes can be switched by altering electrochemical potentials. In a tristable [2]rotaxane composed of a cyclobis(paraquat-p-phenylene) ring and a dumbbell with tetrathiafulvalene, dioxynaphthalene and bipyridinium recognition sites, the position of the ring can be switched. On oxidation, it moves from the tetrathiafulvalene to the dioxynaphthalene, and on reduction, to the bipyridinium radical cation, provided the ring is also reduced simultaneously to the diradical dication.

pH-responsive mechanised nanoparticles gated by semirotaxanes.

A [2]pseudorotaxane-based mechanised nanoparticle system, which operates within an aqueous acidic environment, has been prepared and characterised; this integrated system affords both water-soluble stalk and ring components in an effort to improve the biocompatibility of these promising new drug delivery vehicles.

Mechanised nanoparticles for drug delivery.

Time and time again humanity is faced with a unifying global crisis that crosses the many great divides in different societies and serves to bring once segregated communities back together as a collective whole. This global community instinctively turns to science to develop the means of addressing its most pressing problems. More often than not, these forces dictate the direction that scientific research takes. This influence is no more apparent than in the field of supramolecular chemistry where, for decades now, its responsibility to tackle such issues has been put on the back burner as a consequence of a lack of platforms with which to deliver this contemporary brand of chemistry to meaningful applications. However, the tide is slowly turning as new materials emerge from the field of nanotechnology that are poised to host the many attractive attributes that are inherent in the chemistry of these supermolecules and also in the mechanostereochemistry of mechanically interlocked molecules (MIMs), which can be reused as a sequel to supramolecular chemistry. Mesoporous silica nanoparticles (SNPs) have proven to be supremely effective solid supports as their surfaces are easily functionalised with either supermolecules or MIMs. In turn, the blending of supramolecular chemistry and mechanostereochemistry with mesoporous SNPs has led to a new class of materials - namely, mechanised SNPs that are effectively biological nanoscale 'bombs' that have the potential to infiltrate cells and then, upon the pulling of a chemical trigger, explode! The development of these materials has been driven by the need to devise new therapies for the treatment of cancer. Recent progress in research promises not only to control the acuteness of this widespread and insidious disease, but also to make the harsh treatment less debilitating to patients. This global scourge is the unifying force that has brought together supramolecular chemistry, mechanostereochemistry and nanotechnology, uniting these three communities for the common good. At the nanoscale level, the mechanism for the release of cargos from the confines of the nanopores in the SNPs is accomplished by way of mechanical modifications made on the surface of these functionalised supports. These mechanical motions rely on both supramolecular, i.e., host-guest complexes, and mechanostereochemical phenomena (e.g., bistable rotaxanes), which are often stimulated by changes in pH, light and redox potentials, in addition to enzymatic catalysis. The future of this field lies in the development of 'smart bombs' wherein the loaded mechanised SNPs are endocytosed selectively by cancer cells, whereupon an intracellular trigger causes release of a cytotoxin, effectively leading to apoptosis. This review serves to highlight (1) the evolution of surface-functionalisation of SNPs with supermolecules and also with MIMs, (2) the mechanisms through which controlled-release of cargo from mechanised SNPs occurs, and (3) results from the in vitro application of these mechanised SNPs.