The evolution of biosensors and diagnostic devices has been thriving in its ability to provide reliable tools with simplified operation steps. These evolutions have paved the way for further advances in sensing materials, strategies, and device structures. Polymeric composite materials can be formed into nanostructures and networks of different types, including hydrogels, vesicles, dendrimers, molecularly imprinted polymers (MIP), etc. Due to their biocompatibility, flexibility, and low prices, they are promising tools for future lab-on-chip devices as both manufacturing materials and immobilization surfaces. Polymers can also allow the construction of scaffold materials and 3D structures that further elevate the sensing capabilities of traditional 2D biosensors.
1. Polymer and Biopolymer Nanocomposites
Polymers have been considered prominent candidates for creating an ideal matrix for entrapment and immobilization of biomolecules in the analytical sciences. Characteristics of most polymers, such as high conductivity, ease of biofunctionalization, flexibility, biocompatibility, highly modifiable chemical functions, etc., make them attractive for biosensor development in different fields from environmental analysis to biomedical applications
[1]. The experience with polymeric and co-polymeric materials also demonstrates the successful application of functional polymers for the immobilization of enzymes
[2], micro-organisms
[3], antibodies
[4], and aptamers
[5] in the design of electrochemical biosensors. Historically, polymers have been seen as a practical material choice for electrochemical devices, but recent advances in nanotechnology and the creation of nano-scaled materials has allowed for even further evolution in the biosensor field development due to new intrinsic optical, electrical, and mechanical properties.
Efficient immobilization of bioreceptors (or other components) and optimal signal transduction are crucial for biosensors. Polymers are still key coating matrices for nanomaterials through the fusion of nano-objects and polymers. These combinations have led to the emergence of new nano-scaled hybrid materials or polymer nanocomposites. They have been further employed to construct polymer nanocomposite-based biosensors to obtain highly sensitive and reliable analytical devices by improved catalytical and chemical reactivity, surface specificity, enhanced electrode kinetics, controllable synthesis and morphologies, higher stability, and biocompatibility
[6]. As an alternative to synthetic polymers, some features of biopolymers, such as natural origin, biodegradability, recyclability, lower antigenicity, and their suitable interaction with living systems, make them powerful tools for biosensor fabrication
[7][8].
Polymeric and biopolymeric nanocomposites refer to a hybrid structure in which a polymer matrix is used as a substrate, and nano-scaled organic or inorganic materials are used as fillers. Typically, the polymers of poly(lactic acid) (PLA), poly(ethylene oxide) (PEO), poly(lactic-co-glycolide) (PLG), poly(N-isopropyl acrylamide) (PNIPAM), and polyurethanes, etc. have been utilized as the matrix phase of polymeric nanocomposites. Biopolymeric nanocomposites, which are also called “Bio-nanocomposites”, “bio-hybrids”, and “green nanocomposites” by the popular terms of recent years, are made up of a nanosized additive in naturally occurring polymers including cellulose, chitin, collagen, silk, keratin, alginate, lignin, starch, polyhydroxyalkanoates (PHA), etc.
[9]. The merging of nanosized filler materials into the polymeric matrix produces interesting and improved mechanical, thermal and optical properties. According to this reinforcement strategy, it can be said that filler materials act as molecular bridges enhancing and controlling dimensional stability, flexibility, strength, toughness, durability, thermal stability and conductivity, optical properties (color and transparency), size, distribution, and shape
[7][10][11]. Organic materials (carbon nanotubes (CNTs) and graphene) and inorganic materials (silicates and metal/metal oxides) are the kinds of nanofillers used to prepare nanocomposites made of polymers
[10][11]. The characteristics of polymeric nanocomposites are affected by choice of both the filler and matrix. For instance, while the type of polymer matrix significantly determines the hydrophobicity, transparency, strength, toughness, controlled ionizability, crystallinity, functionality, biocompatibility, and biodegradability, the choice of filler considerably affects the structural and functional properties. Hence, unique polymer nanocomposites can be synthesized by various combinations of nanofillers. This diversity provides application-oriented strategies via selection of filler nano-objects for the desired properties of specific fields, including medicine, diagnostics, biomedical applications, food packaging, optoelectronic devices, biosensing, bioimaging, tissue engineering, cosmetics, energy, etc.
[9][12].
In parallel to their flexible functionalities and fascinating properties, polymeric nanocomposites have been extensively studied to improve sensor performance and have remarkably allowed the fabrication of many novel biosensors in recent years
[13][14][15]. For example, while quantum dots–polymeric nanocomposites exhibit excellent fluorescence properties that can be used in optical biosensors, CNTs–polymeric nanocomposites provide significant enhancement in the mechanical property that can be adapted to an optoelectronic sensing device
[12]. The utilization of polymeric nanocomposites provides needs-based designs. It brings additional key performance parameters, including higher sensitivity and selectivity, lower detection limits, good reproducibility, and stability by providing a large and easily adjustable surface area, higher electrical conductivity, and fast electron transfer rate
[15].
For polymeric nanocomposites, chemists and material scientists have described various synthesis methods, including ion exchange, template synthesis, sol-gel, in-situ polymerization, hydrothermal route, melt intercalation techniques, etc.
[12][16]. The successful design of a polymeric nanocomposite with any required property is a critical step toward the control of interfacial interactions between the nanofiller and the polymer matrix. Understanding the influence of the filler on the size, shape, orientation, dispersion, and compatibility of the polymer matrix is the most important consideration. When creating a new polymer nanocomposite material, an effective formulation is required by considering three main approaches: rationality-based design, functionality-based design, and tailored property-based design. In the design of polymeric nanocomposites, process route, temperature, pressure, and time are the parameters required to be controlled during processing. The nanofiller choice needs to consider the filler shape, size, type, volume, weight, and orientation. In contrast, the matrix preparation needs to consider the kind of polymer, surface nature, and chemistry. Based on the above, the combination of the nanofiller and the polymer matrix must be achieved at a nanoscale level with chemical compatibility and homogenous dispersion.
Polymer nanocomposites are microstructures of hybrid organic–inorganic materials that can be formed into three types; unintercalated (or microcomposite), intercalated (and/or flocculated), or exfoliated (or delaminated). These microstructural forms are controlled by the synthesis method. Of these various synthesis methods, the melt-blending method is eco-friendly because of the lack of solvent usage and is industrially scalable due to its cost effectiveness, however the need for a high temperature that can damage the surface of nanofiller is its main disadvantage. On the other hand, the in-situ polymerization technique provides better exfoliation in comparison to the melt intercalation method. In the case of sol-gel technology, disadvantages such as high temperature, which can cause the degradation and aggregation of polymers, make it uncommon. There are different synthetic methodologies including organic treatment and chemical modifications for the polymer nanocomposites manufacturing process
[17]. Click chemistry and ring opening of epoxides and aziridines are very efficient and common chemical concepts in the fabrication of polymer nanocomposites. In particular, click reactions are versatile coupling methods due to advantages that include methodological simplicity, high reaction yield and high reaction rates, moderate reaction conditions, easily removable byproducts etc. CuAAC click reaction, metal-free click reaction, Diels–alder reaction, and Thiol-ene and thiol-yne reactions are the commonly employed click reactions in the fabrication of polymer nanocomposites
[18]. Additionally, different surface modification strategies have been developed, something which is also a very critical step in biosensing chemistry and design. Surface modification techniques significantly impact the nanocomposites’ structural and functional properties such as reactivity/chemical reactivity, biocompatibility/bioactivity, hydrophobicity, surface energy, dispersion/stability, and surface roughness. Surface modification can be achieved through different reactions with coupling agents and/or surface adsorption and polymeric molecules’ covalent or non-covalent bonding-based grafting. Along with such functionalization techniques, the reported polymer-nanocomposites-modified electrodes are very promising tools to enhance sensing capabilities in terms of high sensitivity and good selectivity for different types of targets such as drugs, heavy metals, pesticides, pathogens, etc. Polymer nanocomposites still hold a great strength in biosensor design since they provide variable morphologies and architectures on electrode surfaces such as films, vesicles, and dendritic structures
[12]. While choosing the nanocomposites-based structural design, the main strategy is to add a nanofiller according to the need to give the final sensor system a targeted feature such as magnetism, fluorescence, electroconductivity, strength etc. The characteristics of a nanofiller considerably affects the properties of the polymer nanocomposites. The incorporation of a nano-scaled structure can add a new feature or improve an existing property. In order to obtain a nanocomposite structure with magnetic properties, magnetic beads can be added to the composite structure, on the other hand quantum dots can be used as nano-modifier to prepare fluorescent polymeric nanocomposites. For the strength enhancement and hydrolytic stability, silica nanoparticles are a very appropriate choice and for their excellent mechanical stability, CNTs are very attractive nanofillers for the polymer nanocomposites. These unique properties, which are gained by adding nanofillers to polymer nanocomposites, significantly increase the analytical performance of the fabricated sensors.
Table 1 represents the common nanofillers and their effect on polymer nanocomposite and advantages for the final properties of the fabricated biosensors.
Table 1. The effect of several nanofillers on the improved properties of polymeric nanocomposites and their sensors.
Nanofiller |
Polymeric Composite |
Effect for the Fabricated Composites |
Advantages of the Sensors |
Ref. |
Nanoclay |
OMMT/PLA |
Improved thermal and mechanical property |
Improved surface morphology and surface reflectance, modified optical properties |
[19] |
Graphene |
GC-COOH |
Electroactivity |
High electroactivity and easy assembly, high sensitivity, |
[20] |
CNT |
Chitosan modified by ferrocene and CNT |
Increased surface area and decreased effective distance between mediator molecules |
Increased recorded analytical signal, and measurement sensitivity |
[21] |
PAMAM dendrimer |
PAMAM-PPy |
Functionality and increased quantity and homogenous distribution of attached biomolecules |
Efficient electron transfer, reversible redox system, and simple reaction procedure |
[22] |
Oleic acid-modified MNPs |
Magnetic cyclodextrin vesicles |
Magnetic property |
Higher sensitivity |
[23] |
Nano rod and Quantum Dot |
TiO2 Nanorod/TiO2 Quantum Dot/Polydopamine |
Strong light absorption and excellent photocatalytic activity |
Stronger photoelectric response under visible light |
[24] |