Enzyme‐Encapsulating Quantum Dot Hydrogels and Xerogels as Biosensors: Multifunctional Platforms for Both Biocatalysis and Fluorescent Probing

Abstract

Quantum dots (QDs) have gained great interest in both fundamental research and technical applications owing to their unique size-dependent physical and electronic properties. [1] The development and the evolution in QD synthesis play a critical role in the progress of QD applications such as biological imaging, photovoltaic and light-emitting devices, and optical sensors. [2] However, many applications and devices are based on nanoparticle assemblies in the solid state instead of nanoparticles in solutions; as a result, nanoparticle assembly is a necessary step in the utilization of nanomaterials. [3] The assembly of QDs as building blocks into functional architectures and the applications of these architectures have been amongst the priorities of QD research. [3b] Gels and aerogels manufactured from silica and metal nanoparticles available in colloidal solutions have recently been proven to provide an opportunity to marry the nanoscale world with that of materials of macro dimensions; these gels can be easily manipulated and processed, whilst maintaining most of the nanoscale properties. Their extremely low density and high porosity provide access to the capacious inner surface of the interconnected nano-objects they consist of and make these gels enormously attractive for applications. [4] In the past few years, great attention is given to the formation of 3D networked QD aerogels. Quantum-confinement effects retain in such large interconnected monoliths, [3,5] thus making them promising candidates for further use in LEDs, photovoltaics, sensors, etc. [6] Particularly, sol-gelderived materials have been used increasingly in the biosensor development, because the pore sizes throughout the meso and macro regimes are appropriate for molecular transport or infiltration of secondary components and also provide potential for encapsulation of a variety of biomaterials from proteins to whole cells. [7] Thus, QD gels will provide suitable media for encapsulating enzymes or other biomolecules. Both biorecognition units (enzymes) and signaling units (QDs) are integrated in such hybrid material, which is essential for the fabrication of QD-based biosensors. However, most reported enzyme-encapsulating materials used in the biosensing field are silica-gel-based and aerogel-based materials that are manufactured through the sol-gel process by addition of biomolecules into the silica precursors. [8] Up to now, only a few reports on QD-gel-based sensors are published, [9] although QDs have been widely utilized as a fluorescence (FL) probe in bioanalysis. [10] The formation of 3D networked QD aerogels through the sol-gel method is mainly based on the partial removal of thiolate ligands by oxidants or the photochemical treatment. [3a, 5b, 11, 12] However, oxidants or irradiation will harm structure and activity of the biomolecules during immobilization in the QD gels. Recently, Kotov and co-workers reported on self-assembly of CdTe nanoparticles under ambient conditions and ultrasound-induced switching between gel and sol. ...

Modern Inorganic Aerogels

“…[74,75] Furthermore they prepared transparent conductive films made of CdSe/ZnS core/shell nanoparticles by using ad estabilization method. [77] Fort his,w e immobilized an enzyme on the surface of the pre-formed gel. [77] Fort his,w e immobilized an enzyme on the surface of the pre-formed gel.…”

Pericellular Hydrogel/Nanonets Inhibit Cancer Cells

“…Like self-assembling peptides and proteins, certain small organic molecules self-assemble [5] in water to afford nanofibrils as matrices of hydrogels [6] (e.g., in response to biostimuli such as enzymes [7] ). Interestingly, a vancomycin-pyrene conjugate, which self-assembles in water to form nanofibrils, [8] exhibits two orders of magnitude enhanced antibacterial activity against vancomycin resistant enterococci (VRE), plausibly through self-assembled multivalent vancomycin binding the receptors on bacterial cell wall.…”

“…1a, our results show that (i) surface and secretory phosphatases [11] from cells catalytically dephosphorylate a small D-peptide derivative (e.g., D- 1 ) to form a hydrogelator (e.g., D- 2 ); (ii) the accumulation of the hydrogelator results in a network of nanofibrils as the scaffold of a hydrogel in the pericellular space; (iii) the pericellular hydrogel/nanonets entrap secretory proteins, block cellular uptake, thus decreasing cell migration, preventing cell adhesion, and induce cell apopotosis; (iv) most importantly, due to the overexpression of surface and secretory phosphatases by cancer cells, [12] the pericellular nanonets selectively form on the cancer cells (e.g., HeLa, MES-SA, and MES-SA/Dx5). As an unexpected example of enzyme-instructed self-assembly [7c] in pericellular space, this work illustrates a new way that controls the fate of different types of cells according to the expression and location of enzymes that regulate the spatiotemporal profiles of molecular nanofibrils.…”

Versatile Aerogels for Sensors

“…There are some other detection methods for the biosensors besides the electrochemical detection, such as fluorescent detection, surface‐enhanced Raman scattering detection (SERS), and electrochemiluminescent detection . For the aerogel‐based fluorescent detection, the general strategy is introduction of fluorescent materials in the 3D network of aerogels to get fluorescent aerogels.…”