Control of Nanopore Wetting by a Photochromic Spiropyran: A Light-Controlled Valve and Electrical Switch

Abstract

By modifying the surface of nanoporous alumina membranes using mixtures of a photochromic spiropyran and hydrophobic molecules, it is possible to control the admission of water into the membrane using light. When the spiropyran is in the thermally stable, relatively hydrophobic closed form, the membrane is not wet by an aqueous solution. Upon exposure to UV light, the spiropyran photoisomerizes to the more polar merocyanine form, allowing water to enter the pores and cross the membrane. Thus, the photosensitive membrane acts as a burst valve, allowing the transport of water and ions across the membrane. If the aqueous solution contains ions, then the membrane acts as an electrical switch; photoisomerization leads to a two-order-of-magnitude increase in ionic conductance, allowing a current to flow across the membrane. Exposure to visible light causes photoisomerization of the merocyanine back to the closed, spiro form, but dewetting of the membrane does not occur spontaneously, due to a high activation barrier.

Electrical Conductance of Hydrophobic Membranes or What Happens below the Surface

“…Reactions (4,5,7,8) have significant equilibrium constants, while reactions (6) and (9) do not proceed at normal pH. For the same reason, intrusion of electrolyte into the pores is more susceptible to low pH when reactions (5,8) are significantly shifted to the right and thus more water can be stabilized near the ionizable groups.…”

“…For the same reason, intrusion of electrolyte into the pores is more susceptible to low pH when reactions (5,8) are significantly shifted to the right and thus more water can be stabilized near the ionizable groups. Other hydrophilic groups in the modifiers such as amides do not contribute as strongly to the membrane conductance because of their ambivalence at neutral pH.…”

“…The mixed SiNH9/SiNspiropyran membranes were prepared using a mixture of decanoic acid and the spiropyran carboxylic acid, 1′-(3-carboxypropyl)-3′,3′-dimethyl-6-nitrospiro[2H-1] benzopyran-2,2′-indoline, in step d of Figure 1 to form a complete monolayer. 5 The amount of bound spiropyran was monitored by its UV absorbance.…”

“…Nevertheless, a membrane with hydrophobic pores will avoid wetting if the pores are of a small size. [1][2][3][4][5]7,8 The latter fact is successfully used in the textile industry. 19 Water intrusion into the pores becomes possible if the pressure gradient at the pore mouth, ΔP, can overcome the surface tension:…”

Fluorescent Dyes Used in Polymer Carriers as Imaging Agents in Anticancer Therapy

“…The design of polymer membranes with controlled pore sizes by using different fluorescent moieties Synthetic membranes containing pores facilitate transport therefore, they are desired in biological applications [173][174][175][176][177][178][179][180]. Recent developments in methods for the preparation of smart responsive polymer membranes and the mechanism of their response to external stimuli with particular attention to the behavior of light responsive polymer membranes was reported in reviews [181,182].…”