多孔膜集成到电化学系统中用于生物分析

Abstract 

Porous membranes have emerged as promising platforms for bioanalysis because of their unique properties including high surface area, selective permeability, and compatibility with electrochemical techniques. This minireview presents an overview of the development and applications of porous membrane-based electrochemical systems for bioanalysis. First, we discuss the existing fabrication methods for porous membranes. Next, we summarize electrochemical detection strategies for bioanalysis using porous membranes. Electrochemical biosensors and cell chips fabricated from porous membranes are discussed as well. Furthermore, porous micro-/nanoneedle devices for bioapplications are described. Finally, the utilization of scanning electrochemical microscopy for cell analysis on porous membranes and electrochemiluminescence sensors is demonstrated. Future perspectives of the described membrane detection strategies and devices are outlined in each section. This work can help enhance the performance of porous membrane-based electrochemical systems and expand the range of their potential applications.

INTRODUCTION 

Bioanalysis is widely conducted for healthcare and environmental assessments. Highly sensitive and selective assays are required to trap, separate, and concentrate analytes. For this purpose, micro/nanofabrication techniques have been successfully used to prepare sophisticated bioanalytical micro/nanodevices such as microfluidic and microwell devices. In addition, porous membranes have been utilized in bioanalysis. A porous membrane consists of a thin layer of holes or pores with diameters  ranging from a few nanometers to tens of micrometers.Porous membranes are used in many applications, such as the separators of lithium-ion batteries, oil/water separation, water desalination,organic liquid separation, ion separation, gas storage and separation, pervaporation, fuel cell technology, electrochemical energy storage,electrochemical metal sensors, and biomedical engineering (including cell culture platforms) owing to their tunable structures, versatile surface chemistry, and large surface areas. In bioanalytical applications, transducers are incorporated into porous membrane systems to detect analytes. Although optical techniques are widely employed for this purpose, electrochemical approaches have attracted considerable attention in bioanalysis because of their simplicity and high sensitivity. This minireview focuses on the electrochemical bioanalysis methods based on porous membranes.

First, we discuss the commonly utilized fabrication techniques for porous membranes and summarize various electrochemical strategies involving porous membranes and electrochemical devices/systems. Next, bioassays that use porous membrane systems containing electrochemical sensors are discussed. Furthermore, this paper highlights the applicability of porous membranes in cell analysis. Finally, the main conclusions from the discussed topics are outlined. Note that this minireview does not focus on detailed assays and devices, such as the effects of pore size/shape and material composition on sensitivity and separation/filtration but demonstrates the concept of bioassays using porous membranes with electrodes.

2 STRATEGIES FOR FABRICATING POROUS MEMBRANES

Polymers,metals,other materials (e.g., carbon, silica, and various oxides),and their composites have been widely used as substrates for porous membranes. Owing to the diversity of synthetic routes and types of polymer materials, polymer-based porous membranes exhibit high versatility in their design and multiple chemical functionalities, which are advantageous over other membranes. Although inorganic membrane materials are more expensive than organic polymeric materials, they possess various advantages such as high-temperature stability, high resistance to solvents, and narrow pore size distribution. Porous membranes are fabricated using top-down approaches (such as track etching and nanoimprinting) and bottom-up techniques (such as block copolymer selfassembly, electrospinning, and 3D bioprinting). Porous paper types have also been utilized for electrochemical bioanalysis.

Track etching-based micro- and nano-fabrication techniques are widely used for preparing porous membranes. An example of the industrial fabrication process for track-etched membranes is presented in Figure 1. The advantage of these techniques is that they can produce a wide range of pore densities and sizes varying from the nanoscale to the microscale. Track etching technology is based on irradiating polymers with energetic heavy ions, electrons, X-rays, or ultraviolet light, forming linearly damaged tracks through the exposed polymeric film. Polyvinylidene fluoride, polyester, and polycarbonate are commonly used materials in this approach. The damaged tracks are transferred into the pores using either appropriately selected wet chemical etching conditions or an applied electric field.

Fig1

Nano-/microimprint lithography is used to produce porous polymeric membranes. It is a mechanical lithography technique that involves pressing a stamp or template against a deformable resist layer deposited on a substrate, such as a silicon wafer or metallic surface. The fabrication of a master mold is one of the most critical steps as the accuracy and definition of its structure are essential for defining the pattern of imprinted polymers. Templates or master molds are often fabricated via photolithography or electron beam lithography (at the nanoscale) to create controlled nano- and microstructural patterns.

3 ELECTROCHEMICAL DETECTION STRATEGIES USING POROUS MEMBRANES

In addition to redox currents, ion currents flowing through membranes were used as bioanalysis indicators (Figure 2F). In this strategy, a pair of electrodes are set at the top and bottom of the membrane, and a bias is applied between the electrodes to monitor ion currents. When analytes are trapped inside the pores, they block the ion current flow, resulting in their successful detection. Furthermore, the ion flow is used for chemical delivery, as described below in the “Porous micro/nanoneedle devices for bioapplications” section.

Fig2

4 ELECTROCHEMICAL BIOSENSORS USING POROUS MEMBRANES MODIFIED WITH BIORECOGNITION ELEMENTS

Biorecognition elements such as antibodies, DNA, peptides, enzymes, and aptamers are used in porous membrane sensors because they can effectively trap analytes for electrochemical detection. As an example of the antibodybased immunosensors, nanoporous anodic alumina membranes modified with Flightless I antibody recognition sites were used to detect Flightless I, a biomarker of wound chronicity (Figure 3A).During the detection procedure, a working electrode was placed on the membrane surface. When Flightless I was captured inside the nanochannels, the diffusion of [Fe(CN)6] 4− from the bulk to the electrode was blocked. The amount of Flightless I was quantified by suppressing redox currents; therefore, the developed system was a label-free detection platform. This concept has been widely utilized in immunoassays. For example, a nanoporous anodic alumina membrane was employed to detect viral particles via immunoreactions.

Fig3

Herein, we reviewed various porous membrane-based electrochemical systems for bioanalysis. Their unique properties combined with electrochemical techniques enable a wide range of applications in biomolecule separation, sensing, and drug delivery. The development of advanced fabrication methods and novel electrochemically active materials can further enhance the performance of porous membrane-based electrochemical systems and expand the range of their potential applications.