Nanoporous membranes: A new prospective

1. INTRODUCTION

On the basis of geometrical shape of pores we can distinguish nanopores membrane in various types such as cylindrical, conical, biconical, and bullet. In addition, we can distinguish membrane with respect to the type of material used in foils. In general, we can synthesize two types of membrane: (i) Single pore membrane (ii) Multi-pores membrane. Both types of membrane have their individual use in various fields of research as well as in general life.

The transport using nanoporous (multi-nanopores) membranes used in various applications such as instance protein separation and purification, drug delivery and biomedical applications etc. [1]. In order for the membrane to be applied in membrane technology, it is necessary to make membrane from foils according to their use.

For the fabrication of anodized membranes, generally used materials are anodic aluminum

aluminum substrates in an aqueous acidic solution [2]. The geometrical condition of nanopores can be well-ordered by tuning the anodization process. The nanopores in AAO membranes can be produced with 10–200 nm in diameter, pore length about several tens of μm and pore density of the order of 1011

pores cm-2 with the help of chemical mechanisms in the fabrication process.

Microfacricated membranes are synthetized with micro- and nano-fabrication processes. By this method we can manufacture cylindrical arrays of nanopores in silicon nitride (Si3N4) and silicon (Si) membranes. For producing an array of uniform cylindrical nanopores we can used focused ion beam (FIB) etching technique [3] in an ultrathin silicon nitride membrane. The production of silicon nitride and silicon membranes are costly and this is the main drawback of this technology.

Ion-track technique is the method employed for the production of nanoporous polymeric membranes. With this technique, we can quite easily control the pore size as well as pore geometry by changing the external conditions in etching process (etching time, temperature and etchant concentration). The density of the pores per unit area can be controlled by adjusting the density of projectile ions (ion fluence). The commonly used track-etched membranes include PC, PET, and PI of thickness ranging from 5-100 μm. Track-etched nanopores are randomly distributed and normally cylindrical or conical in geometry. In polymer nanopores membrane, a conical nanopore have different diameters on both sides but cylindrical nanopore have the same diameter shown in Fig. 1. (c) and (d). Similarly other nanopores shape mainly changed their diameter as shown in Fig. 1. (e) and (f).

Biological nanopores are used in various applications in research of biophysics, biochemistry, and chemistry for sensing, detecting of individual atom and molecule [4]. Due to their different geometry and chemical configuration, nanopores are capable for utilizing in biological application for detection of atoms and molecules. Solid-state nanopores are generally called synthetic nanopores in recent days. These nanopores membranes are the substitute of biological nanopores. This is due to the fact that they molecule is recorded by current changes as they translocate through the pore.

Glass nanopipettes are used for molecule sensing. In the fabrication process of glass nanopipettes, pipette pullers are generally used. Now a days the pipette pullers are commercially available. In synthetic process glass capillary tube is first heated with the laser. After that glass tube can be mechanically pulled until it breaks at the narrow neck, thereby forming two open-ended nanopipettes. In this process we can produce nanopipettes with opening diameter of several tens of nanometers [5].

For the purpose of DNA sequencing the electric field drive DNA molecules through nanopores and by using the time characteristic of nanopore ion current distinguish the size of DNA. This method can be divided into two types of materials (i) bio-nanopores DNA sequencing, and (ii) solid-state nanopores DNA sequencing [6]. The main disadvantage of bio-nanopore sequencing is that it pause and reverse DNA molecules which makes noise in the current-time signal and the signal misinterpreted [7]. But DNA sequencing with solid-state nanopores sensors can realize the parallel detection of DNA [8, 9]. In addition, solid-state nanopores DNA sensors are cost effective.

Recently, the short diameter ~3nm solid-state nanopores successfully detected the ubiquitin protein and analyzed the connection between ubiquitin protein and protein. Solid state nanopore possesses high detection resolution for the molecule internal structure. Nanopore sensing and detection technology have been used in real-time detection of DNA protein interactions, protein-protein interactions, detection and diagnosis of diseases and detection of heavy metal ions and viruses.

Energy conversion methods based on nanopores convert the energy existing in environment, such as mechanical, chemical, light, and electric into other forms and does not produce carbon dioxide, vibrations and noise[10,11]. The function of ion channels of cell membrane is the main inspiration of energy conversion with nanopores [12]. Due to the excellent performance of nanopores, such as tunable size and shape, chemical durability, thermostability, etc. [13], they can used in energy conversion. Besides these practical applications,

extraordinary performance in energy conversion [14, 15].

As the asymmetric current flow in nanofluidic devices, it is comparable to the behavior of electronic diodes [16] and transistors [17]. The exception is that in nanofluidic devices ions of solution rather than electrons work as the charge carriers [18]. Implanted electrodes near the nanopores with a concentration gradient solution act as a controllable gate and effectively alter the flow of current through the nanopores. In addition, tunable voltage influences fluid transport and ionic current rectification [19].

Several mechanisms to separate the molecules are used now a days with improved resolution and separation speed. By using nanofluidic separations mechanism molecules are separated on the basis of charge with significantly reduced separation lengths [20]. For the separation of two fluorescent molecules, the shear-driven flow mechanisms used in nanopores generates high flow velocities for high-speed chromatographic separations [21].

Resistive pulse sensing (RPS) is a label-free, nondestructive method to detect/ analyzed individual particles/molecules. Transient decrement in ionic current, or pulses observed when particles pass through an electrically biased pores/nanofludic devices and are proportional to particle size. Track etched nanopores technique have the ability to tune the pores size and shape and this features has increased the types of nanoscale particles that can be sensed[22].

Template-based approaches combined with electrodeposition technique has been used to produce nanowires of different size and shape embedded nanopores of track-etched polymeric membranes. The nanoporous structures make it possible to tailor specific nanowires structures. The prepared nanowires were used in the fields including, nanoelectronics, plasmons, biotechnology, thermoelectrics, solar cells, sensors, catalysis etc [23].

Nanopores membranes are much effective now a day. These can be prepared with natural as well as artificial techniques. These are used in various

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Materials Science Lab., Department of Physics, Chaudhary Devi Lal University, Sirsa-125055, India