Nafion (Nf)

Conducting polymers which also known as intrinsically conducting polymers (ICPs) are a new class of polymers that possesses a high electron affinity and high electronic conductivity by several orders of magnitude of doping (while maintaining mechanical flexibility and high thermal stability). These can be associated to the π-electron backbone owned by ICPs. ICPs have been long synthesized and used for the various applications including electrochemical biosensor. One of the advantages possess by ICPs is that it is easy to synthesize through chemical or electrochemical process which produces a stable and reproducible dispersion. This has encourages researchers to employ ICPs material in the fabrication of sensors especially for detecting biomolecules. Other remarkable advantages of using ICPs is that it provides an excellent signal transduction for molecular detection due to its high sensitivity in their conductivity and electrochemical activity towards molecular interaction, thus renders possibilities for sensor application (Janata et al., 2003). Moreover, ICPs can act as an electron promoter that are compatible with biological molecules in neutral aqueous solutions (Mulchandani et al., 1996). Besides that, the ICPs itself can be modified to

bind biomolecules to a sensor, thus able to increase the sensitivity of the sensor (Rahman et al., 2008).

Some of the ICPs groups that are frequently used in sensors application are polyacetylene (Zeng et al., 2009), polythiophene (Védrine et al., 2003), poly(p- phenylenevinylene) (Srinivas et al., 2012), poly(3,4-ethylenedioxythiophene) (Nien et

al., 2006), polypyrrole (Uang et al., 2003) and polyaniline (Langer et al., 2004). Among

them, Nf is one of the most extensively used ICPs in the design of bioanalytical sensors. Nf is a sulfonated tetrafluoroethylene based fluoropolymer-copolymer with a conductive properties. Due to the presence of perfluoroalkyl backbones in Nf, it has high hydrophobicity properties hence making it an effective matrix to disperse graphene in an aqueous solution as graphene tend to agglomerate or even restack to form graphite through strong π-π stacking and van der Waal’s interaction (Yin et al., 2010). In sensor application, Nf was used as an electrode modifier for sensor electrode fabrication due to its antifouling capacity, chemical inertness and high permeability to cations (Zarei et al., 2015).

Besides that, Nf also helps to enhance the stability of graphene modified electrodes due to its excellent film forming ability. As a cation exchange polymer, Nf helps to blocks the anionic species from reaching the electrode surfaces and allows the cation conduction to pass through, thus leads to good selectivity (Chou et al., 2009). Interestingly, the hydrophilic negatively charged sulfonate group in Nf film enables selective pre-concentration of positively charged biomolecules through electrostatic interaction, whereas, the hydrophobic fluorocarbon network of the polymer gives a selectivity for the hydrophobic part of the molecule (Nigović et al., 2014a). These advantages making Nf an ideal choice for the fabrication of electrochemical sensors.

The comparison of various conducting polymer-based modified electrodes for the detection of different bioanalytes by using electrochemical technique has been listed in Table 2.3.

Table 2.3: Comparison of various conducting polymer-based modified electrodes for

the detection of different bioanalytes using electrochemical technique.

Electrode Target

analyte Analytical method Linear range Detection limit Reference

CPPDAN NE DPV 9.90 -

90.9 µM

1.82 µM (Guedes da Silva et al., 2011)

PAH/SPE NADH Amperometry 0.01 - 5

mM 0.22 µM (Rotariu et al., 2014) nanoporous PEDOT/ GCE NADH Amperometry 5 - 45 µM 3.8 µM (Rajaram et al., 2015) Nf/p(FcAni )-CNTsPE DA DPV 150 μM 1- 0.21 µM (Sroysee et al.) Pty/GCE DA LSSV 1-7 µM 161 nM (Khudaish et al., 2012)

CPPDAN=Cathodically Pretreated Poly(1,5-diaminonaphthalene); NE=norepinephrine; NADH=nicotinamide adenine dinucleotide; PAH=Poly(allylamine hydrochloride); SPE=screen-printed carbon electrode; PEDOT=poly(3,4)ethylenedioxythiophene; p(FcAni)=poly(m-ferrocenylaniline; CNTsPE=carbon nanotubes-paste electrode; Pty=polytyramine; LSSV=linear square stripping voltammetry.

2.3.3 Metal Nanoparticles

Metal nanoparticles such as gold (Au), silver (Ag), platinum (Pt), palladium (Pd), copper (Cu), zinc (Zn) and etc, have attracted much attention because of their extraordinary properties in different fields of optics (Augustine et al., 2014), optoelectronics (Borsella et al., 1999; Conoci et al., 2006), catalysts (Lesiak et al., 2014; Li et al., 2014), solar cell (Hai et al., 2013; Kang et al., 2010) and sensors (Li et

al., 2014). The unique chemical and physical properties of metal nanoparticles make it

potentially useful for designing new and improved sensing devices, especially for electrochemical sensors. Their excellent electrocatalytic properties and high load

employed as electrochemical signal enhancer in sensor application. With regard to this, silver nanoparticles (AgNPs) have been extensively investigated as an effective electrocatalyst for electrochemical sensor applications. AgNPs keep on being of enormous current research attention because it required low production cost, environmentally friendly, low toxicity and biocompatibility. Moreover, AgNPs possessed the advantages of excellent catalytic activity, high conductivity and high surface energy, which makes them a promising catalyst material. Furthermore, their high surface to volume ratio allows large fraction of metal atoms to get exposed to reactant molecules and it is very much desirable for sensor application (Rastogi et al., 2014). Besides that, it is also the best conductor among all of the noble metals (Jiang et

al., 2013). Due to these properties, AgNPs may facilitate more efficient electron transfer

than the other noble metal nanoparticles.

Besides AgNPs, gold nanoparticles (AuNPs) also have recently drawing an increasing attention of many researchers in the field of sensors. AuNPs have attracted more attention from researcher due to its novel chemical, optical and physical properties such as high effective surface to volume ratio, excellent electrical and heat conductivity, and strong absorption in the visible and near infrared wavelength region (380 to 750 nm). Important physical properties of AuNPs include surface plasmon resonance (SPR) and the ability to quench fluorescence (Yeh et al., 2012). Besides that, it is also have an excellent biocompatibility and low toxicity which make is suitable to be applied in biotechnology (Khlebtsov et al., 2011). AuNPs also exhibit high chemical stability and inertness under physiological conditions as well as possesses excellent electrocatalysis properties. All these properties make AuNPs an attractive material for electrochemical and biological devices. More interestingly, the properties of Au can be controlled by tuning the shape and size (Jain et al., 2006). Because of its small size, Au

could provide high active surface area, thus, improve the electron transfer. This will lead to the enhancement in sensitivity and signal to noise ratio, therefore improve the analytical performance.

Another noble metal that has the potential as a catalyst for the electrochemical sensor is palladium nanoparticles (PdNPs). PdNPs have attracted extensive attention because of their good chemical and physical properties, including its wear and corrosion resistance as well as good stability. Its high specific surface areas would increase the mass transport and enhance the electron transfer kinetics, thus, improves the electrocatalytic activity. Besides that, PdNPs is also an important material in applications involving hydrogen storage and gas sensing due to its ability to absorb hydrogen at high capacity (Tobiška et al., 2001). Table 2.4 compiles the analytical parameters for electrochemical sensing of biomolecules by using various metals nanoparticles as the sensor electrode that have been reported in literature.

Table 2.4: Comparison of various metals nanoparticles-based modified electrodes for

the detection of different bioanalytes by using electrochemical technique.

Electrode Target

analyte Analytical method Linear range Detection limit Reference

AgNPs/

MCPE DA DPV 1 – 5 µM 0.085 μM (Vidya et al., 2016)

ITO/ AuNPs DA DPV 1×10 −7 - 9×10−4 M 34.5 µM (Aldana- González et al., 2013) Ag–

Pd/GCE L-Cys CV - 2.8 µM (Murugavelu et al., 2014)

Fe@Pt/C H2O2 Amperometry 2.5 μM-

41.605 mM 750 nM (Mei et al., 2016) Au/ITO 5-HT SWV 1.0×10−8– 2.5×10−4 M 3 nM (Goyal et al., 2007)

AgNPs=silver nanoparticles; MCPE=modified carbon paste electrode; ITO=indium tin oxide; Pd=palladium; CV=cyclic voltammetry; Fe=ferum; Pt=platinum; C=carbon; 5-HT=serotonin;