Influence of Nanoconfinement on the pKa of Polyelectrolyte Functionalized Silica Mesopores

Abstract

a variety of applications, such as drug delivery or biocatalysis accessible. [2] In addition, mesoporous silica offers a wide variety of transport characteristics achieved by introducing organic functions into the silica framework. Those silica hybrid materials are a common motive in separation processes, drug delivery, or sensor technology. [3] Key properties, such as high specific surface area, stability, adjustable pore geometries, as well as high diversity regarding surface chemistry allow their application in such diverse areas. [4] Especially the responsive, so-called smart, organo-silica materials have been established as a new fascinating field of research over the last decades. [5] Using responsive polymers, gates are created in silica nanopores which react to triggers, such as light, [6] pH, [7] or temperature [8] and thus control pore accessibility, transport, or the release of drugs. [9] Applications for mesoporous silica strongly depend on charge generation and the corresponding gating behavior of the mesopores, which result from interactions with pH adjusted solutions.Controlled polymerization and thus precise control on pore filling and charge density in nanopores has been demonstrated by atom transfer radical polymerization, reversible additionfragmentation chain transfer, or surface initiated photoiniferter polymerization. [10] Only very recently it has been shown that not only the amount of polymer but also the architecture of the polymer chains can be controlled and block-cooligomers in pores can be generated. [10a,11] Thereby, pH responsive polymers Functionalized ordered mesoporous materials are relevant in technologies, such as drug release, sensing, and separation. To design functionality, the silica framework can be functionalized with responsive molecules or polymers. Often, the pH value in those hybrid materials determines performance. Even though pH/pKa differences between polymers in bulk solutions and nanoscale confinement have been observed, the influence of confinement on pH-and pore filling dependent polyelectrolyte oligomer chain charge has yet not been investigated systematically. Here, mesoporous silica films are functionalized with (2-dimethylamino) ethyl methacrylate) (DMAEMA) and 2-(methacryloyloxy)ethyl phosphate (MEP) oligomers using photoiniferter initiated polymerization. This approach allows a controlled and environmentally friendly mesopore functionalization in water. The obtained oligomer functionalized pores are tunable with respect to pore filling. For both, poly(2-(dimethylamino) ethyl methacrylate) (PDMAEMA) and poly(2-(methacryloxy)ethyl phosphate) (PMEP), the charge generation inside mesopore confinement is significantly delayed toward harsher pH conditions resulting in pKa shifts of 1-2 pH units. Polymer amount and ionic strength show to further influence the pKa of PDMAEMA in mesopores. The technological importance of the pH value in confinement and its effect on enzyme stabilization is demonstrated. Lipase from Aspergillus oryzae loses its activity upo...

The Interplay of Nanoconfinement and pH from the Perspective of a Dye‐Reporter Molecule

“…These findings using MPT−Ph as a new pH‐reporter dye molecule sensitive to the required pH region (pH 1–5) in which mesoporous silica responds are in accordance with previous findings from our group investigating the pKa value of short oligomer chains in silica mesopores as well as with theoretical simulations for short polymer chains in ion channels [9a,11a] …”

“…Two different mesoporous silica materials with comparable porous structures were used as frameworks to incorporate the MPT−Ph dye. Mesoporous silica thin films with pore diameters of 8 and 16 nm and film thicknesses of 200–1000 nm were prepared using sol‐gel chemistry and evaporation‐induced self‐assembly as previously reported [9a,20] . Additionally, amino‐functionalized mesoporous silica SBA‐15 (Santa Barbara amorphous 15) particles were prepared by a previously reported strategy [21] .…”

“…In this context, differences between the pH value in solution and under nanoconfined conditions, for example, within silica mesopores, have been reported based on theoretical studies and indirect experimental observations [11] . To date, experimental studies have been limited to indirect measurements such as ion flow through nanochannels or mesopore permselectivity to determine the charge of molecules inside nanoscale environments [7,9a,12] . Although pH detection in MSMs has gained more importance in recent years, the direct measurement of the apparent pH value inside nanopores has not yet been fully explored [11b,13] .…”

Surface Plasmons and Visible Light Iniferter Initiated Polymerization for Nanolocal Functionalization of Mesoporous Separation Layers

“…Dimethylaminoethyl methacrylate (DMAEMA) was used as monomer due to its responsivity to pH and reported transport regulation. [11] Polymerizations were conducted under visible light irradiation without additional radical sources or catalysts in 600-800 nm thick mesoporous silica films with pore sizes of 12 nm (neck) and 16 nm ( Figure S4, Supporting Information).…”

Small Molecules, Non-Covalent Interactions, and Confinement

“…PMS-type materials are ideal host systems for confinement studies since they combine narrow pore-diameter distributions and large specific surface with good chemical stability, easy handling, and chemical functionalization. They were employed in studies on the structural and dynamic properties of many different confined molecules, including water, alcohols, carbonic acids, protein solutions, and on thermophysical processes, such as freezing and melting points, glass transitions [ 23 , 24 , 25 , 26 , 27 , 28 , 29 ], or an electrochemical study of local pKa in confinement [ 30 ].…”