Development of porous membranes capable of controlling flow or changing their permeability to specific chemical entities, in response to small changes in environmental stimuli, is an area of appealing research, since these membranes present a wide variety of applications. The synthesis of these membranes has been mainly approached through grafting of environmentally responsive polymers to the surface walls of polymeric porous membranes. This synergizes the chemical stability and mechanical strength of the polymer membrane with the fast response times of the bonded polymer chains. Therefore, different composite membranes capable of changing their effective pore size with environmental triggers have been developed. A recent interest has been the development of porous membranes responsive to light, since these can achieve rapid, remote, noninvasive, and localized flow control. This work describes the synthesis pathway to construct intelligent optothermally responsive membranes. The method followed involved the grafting of optothermally responsive polymer-metal nanoparticle nanocomposites to polycarbonate track-etched porous membranes (PCTEPMs). The nanoparticles coupled to the polymer grafts serve as the optothermal energy converters to achieve optical switching of the pores. The results of the paper show that grafting of the polymer and
Porous membranes capable of controlling flow or changing their permeability to specific chemical entities, in response to small changes in environmental stimuli, are an area of appealing research, since these membranes present potential applications as sensors, in bioseparations, as drug delivery systems, and as valves that serve to interconnect microfluidic systems and control interflow [
Different methods have been reported in the literature to achieve grafting of polymer chains to the surface of porous membranes and these techniques fall predominantly into two categories: “grafting to” and “grafting from.” We found in the literature that growing polymers from the surface, “grafting from,” rather than functionalizing the polymers, “grafting to,” allows better control on reaction and permits obtaining higher polymer densities [
For the grafting of PNIPAM to the PCTEPM we employed a plasma-induced technique that has been previously reported in the literature [
The plasma-induced grafting presents a major advantage for our synthesis since it can achieve a control over the location of the grafting of the PNIPAM chains, either on the membrane surface or to the walls of the pores [
We controlled the grafting location of the PNIPAM by wetting the membrane using ethanol prior to the initiation of the surface polymerization. It has been shown previously that ethanol allows a more efficient wetting of the PCTEPM and so promotes grafting of the polymer in the pores [
Show scanning electron microscopy micrographs of the polycarbonate porous membranes. (a) Control PCTEPM surface without PNIPAM grafts. (b) Clean cut internal structure of the control PCTEPM without PNIPAM grafts. (c) Surface and (d) internal structure of PCTEPM that has been grafted with PNIPAM.
The coupling of metal particles to the PNIPAM grafts in the PCTEPM is approached through an
The localization of the polymer grafts on each PCTEPM sample and the size and shape of the synthesized nanoparticles were analyzed using SEM. As a control of the
Show scanning electron microscopy micrographs of the porous polycarbonate membranes. (a) Control PCTEPM surface without PNIPAM grafts after silver nanoparticles have been synthesized in
From Figures
(a) Size distribution of the silver nanoparticles synthesized
The PNIPAM chains in an aqueous medium grafted to the surface of the PCTEPM, when exposed to temperature, are expected to act as a mechanical valve that controls flow. We tested the temperature dependent average flows through the porous membranes using gravity-driven flow with time changing hydrostatic pressure starting at 500 Pa and ending at 200 Pa, since the water column decreases as the water flows. Membranes with and without silver nanoparticles were tested for three different switching cycles and produced reproducible results, meaning the polymer and nanocomposites were grafted stably to the membranes. Figure
Permeability switching of gPCTEPMs in response to (a) heat and (b) light. Circles correspond to the control ungrafted membrane in (a) and grafted without nanoparticles in (b). Squares correspond to grafted membrane in (a) and grafted membrane with silver nanoparticles in (b).
Studies with track-etched membranes often calculate effective pore diameters based on Hagen-Poiseuille’s law for the flow rate in a cylinder:
Despite the shortcomings of the Hagen-Poiseuille approximation, the pore size changes observed for the membranes with grafts polymerized are shown in Figures
The
A very similar experimental setup and pore size calculations (Hagen Poiseulle) used for the thermal switching experiments were used for the light-induced permeability changes in the functionalized porous membranes (Figure
Deionized water with a resistivity of at least (18.0 MΩ cm) (E-Pure, Barnstead Thermolyne) was used in all experiments. N-Isopropyl acrylamide (NIPAM) (Fisher Scientific) was recrystallized once in hexane (Fisher Scientific) and stored at −20°C until use. Ascorbic acid (AsA) (Fisher Scientific) and silver nitrate (AgNO3) (Sigma Aldrich) were used as purchased. Polycarbonate track-etched (PCTE) membranes 400 nm and 60
Linear PNIPAM chains were grafted to both the surface and into the pores of the PCTEPM of 400 nm diameter by a plasma-induced grafting polymerization technique reported by Xie et al. [
We achieved the growth of silver nanoparticles in the pores by two approaches: the first involved the following steps. The PCTEPM with surface-grafted PNIPAM are submerged into a 5 mL aqueous solution of AgNO3 (0.001 M) and left for 1 hr to impregnate. Afterward AsA is added to the 5 mL aqueous solution to achieve a 0.003 M solution, and the solution is stirred for 10 minutes. Finally, the membranes are rinsed with deionized water and sonicated for 10 minutes to remove any particles that are not within the PNIPAM grafts.
As a second approach, in order to drive the synthesis of silver nanoparticles inside pores of the membrane, a PNIPAM grafted membrane was taped to the end of the thicker part of a Pasteur pipette and submerged into a 5 mL aqueous solution of AsA (0.03 M) contained in a 10 mL volume container. Immediately afterwards an aqueous solution of AgNO3 (0.01 M) was poured into the cylindrical tube, making sure there was no positive or negative pressure from the column of the aqueous solution of AgNO3. This allowed both the AsA and AgNO3 to diffuse in opposite directions inside the membrane, due to concentration gradients, and allowed nanoparticle reduction within the pores.
A size distribution analysis of the particles at the different synthesis conditions was then carried out through the analysis of different SEM micrographs. The distribution was obtained by analyzing the size of 400 particles chosen at random. With the data on particle size, average particle size and standard deviation (stdev) were determined.
SEM images were obtained with a LEO 1530 scanning electron microscope at an acceleration voltage of 10 kV. The specimens were prepared placing the membranes on a carbon-coated conducting tape and then the samples were coated unless specified at each image shown with silver, which allowed better quality images and higher resolution. Clean-cut images, that allowed analysis of the interior of the membrane, were obtained by breaking the membrane while it was submerged into liquid nitrogen. The membrane was soaked in water prior to putting it into liquid nitrogen.
Permeability changes in the membrane induced by temperature were determined by measuring water flow through the membranes for three different switching cycles. The membrane was taped to the end of a Pasteur pipette (diameter of 5 mm) containing a water column of 5 cm (~1 mL). Afterwards, 0.43 mL was allowed to flow through the membranes. By the setup of the experiment, the differential pressure was not kept constant, since the water column changed height as the flow passed. The respective times were recorded and the average flow rate was determined every 20 s. The volume of water flowing through the membrane was collected in a tube and differences in height were determined every 20 seconds. Afterwards, the differences in height were converted to volumetric flow (mL/s). Additionally, the times for the sample membrane were normalized to an untreated PCTEPM for converting flow rates using the Hagen-Poiseuille equation to an effective pore size. The temperature switching experiments of the membranes were performed by heating the volume of flowing water to different temperatures between 27°C and 40°C.
For the experiments to test water flow control in membranes using light, the same setup and procedure that were followed for testing thermal control of flow were followed. However, instead of temperature, the light switching experiments of the membranes were performed by exposing the membrane to a laser beam of light at a wavelength of 430 nm for 160 seconds. The flowing water was previously heated to 31°C and the average water flow was recorded in response to time. The water flow measurements initiated at the time the membrane was exposed to the laser beam and until the laser was turned off and the flow returned to the initial conditions.
We successfully developed a synthesis method to incorporate PNIPAM-metal grafts into polycarbonate porous membranes to create responsive valves. The synthesis involved initial grafting of the PNIPAM followed by reduction
The synthesized membranes showed, experimentally, a switch in response to temperature and light and achieved differences in flow. Therefore this synthesis methodology is a step forward to the development of responsive membranes with potential applications as optically responsive valves for the spatiotemporal delivery of bioactive agents, cell array, and advanced cell culture applications, among others.
The author declares that there is no conflict of interests regarding the publication of this paper.
This project was funded through PAICyT and PROVERICIT from the Universidad Autonoma de Nuevo Leon. This project was also supported by Paicyt, with support from the Universidad Autónoma de Nuevo León. The authors would like to thank the University for the funding.