Metallic nanoparticles biosensor applications

The lure of new physical phenomena and new patterns of chemical reactivity has driven a tremendous surge in the study of nanoscale materials. This activity spans many areas of chemistry. In the specific field of electrochemistry, much of the activity has focused on several areas (a) electrocatalysis with nanoparticles (NPs) of metals supported on various substrates, for example, fuel-cell catalysts comprising Pt or Ag NPs supported on carbon [1,2], (b) the fundamental electrochemical behavior of NPs of noble metals, for example, quantized double-layer charging of thiol-capped Au NPs [3-5], (c) the electrochemical and photoelectrochemical behavior of semiconductor NPs [4, 6-8], and (d) biosensor applications of nanoparticles [9, 10]. These topics have received much attention, and relatively recent reviews of these areas are cited. Considerably less has been reported on the fundamental electrochemical behavior of electroactive NPs that do not fall within these categories. In particular, work is only beginning in the area of the electrochemistry of discrete, electroactive NPs. That is the topic of this review, which discusses the synthesis, interfacial immobilization and electrochemical behavior of electroactive NPs. The review is not intended to be an exhaustive treatment of the area, but rather to give a flavor of the types of systems that have been examined and the types of phenomena that can influence the electrochemical behavior of electroactive NPs. 

While the variety of NPs used in catalytic and sensor applications is extensive, this chapter will primarily focus on metallic and semiconductor NPs. The term functional nanoparticle will refer to a nanoparticle that interacts with a complementary molecule and facilitate an electrochemical process, integrating supramolecular and redox function. The chapter will first concentrate on the role of exo-active surfaces and core-based materials within sensor applications. Exo-active surfaces will be evaluated based upon their types of molecular receptors, ability to incorporate multiple chemical functionalities, selectivity toward distinct analytes, versatility as nanoscale receptors, and ability to modify electrodes via nanocomposite assemblies. Core-based materials will focus on electrochemical labeling and tagging methods for biosensor applications, as well as biological processes that generate an electrochemical response at their core. Finally, this chapter will shift its focus toward the catalytic nature of NPs, discussing electrochemical reactions and enhancement in electron transfer. 

The development of assemblies of inorganic materials with biomolecules has emerged as a novel approach to the controlled fabrication of functionalized nanostructures and networks.5 The practice of DNA sequence detection is especially relevant for forensic sciences, food safety, genetics and other fields.6 The immobilization of single strand DNA probes onto solid materials such as noble metal nanoparticles has proved to be the basis for a multitude of quite different nanobiotech-nological and biomedical applications, including the DNA driven assembly of nanoparticles and biosensors.5-11... 

Electrospray has been used for many different applications, such as the deposition of paints and coatings on metal surfaces and the deposition of metal nanoparticles and biomolecules on biosensor surfaces, and in a miniaturized version also as a propulsion mechanism in microsatellites (see also the section on electric wind). One particularly interesting application is in fuel atomization, that is, a finer fuel aerosol and atomization will give a higher combustion efficiency and less pollutant emission, which is caused by the effect that finer droplets increase the total surface area on which combustion can start (Lehr and Hiller, 1993). 

These electromagnetic waves are very sensitive to any change in the boundary—for example, to the adsorption of molecules onto the metal surface. SPR has measured the absorption of material onto planar metal surfaces (typically Au, Ag, Cu, Ti, or Cr) or onto metal nanoparticles and is used in many color-based biosensor applications and lab-on-a-chip sensors. To observe SPR, the complex dielectric constants e1 of the metal and s2 of the dielectric (glass or air) must satisfy the conditions Re(ei) < 0 and > e21,... 

Among noble-metal nanoparticles, AuNPs have been the most extensively used in electrochemical biosensor applications. This is because the biochemical activity of the labeled receptor biomolecules (i.e., proteins and DNA among others) is retained when AuNPs are coupled to them (25-27). Particularly, AuNPs have been successfully used as electroactive label in the detection of DNA sequences, based on the highly specific hybridization of complementary strands of DNA (2, 5, 28-31). 

One of the most applicable metal nanoparticles is gold nanoparticles which have been used in different fields. These applications are enzymatic biosensors based on gold nanoparticles and their applications in genosensors, immunosen-sors, and electrocatalytic chemosensors (Fredy, 2008). 

Enzyme sensors are another important application of metal nanoparticles in CMEs besides nonenzyme sensors. Many enzymes can keep their activity when anchored onto gold nanoparticles. A novel method for fabrication of a biosensor based on the combination of sol-gel and self-assembled techniques has been introduced very recently. For example, the gold nanoparticles and enzyme horseradish peroxidase (HRP) can be successfully immobilized on gold electrode by the help of sol-gel with thiol groups, and the direct electrochemistry of HRP has been achieved and the biosensor thus prepared exhibits fast response, good reproducibility, and long-term stability. 

Metallic nanoparticles are very interesting materials with unique electronic and electrocatalytic properties which depends on their size and morphology [63, 64]. The efiftciency of electronic and electrochemical redox properties becomes these classes of nanostructured materials very interesting for technological applications. In particular, gold nanoparticles (AuNPs) are much explored materials as components for biosensors development due to the capability to increase electronic signal when a biological component is maintained in contact with nanostructured surface. On the other hand, silver, platinum, palladium, cooper, cobalt and others... 

Nowadays, to increase sensitivity and selectivity of electrochemical applications, besides new techniques, chemical modification and functionalization of electrodes have also been conducted (Katz et al., 2004). In recent years, new electrode materials like GCPE and bismuth film electrode (BiFE) have been developed and applied to electrochemical biosensor systems (Anik et al., 2008 Timur and Anik, 2007 Wang et al., 2001). As mentioned earlier, nanomaterials like carbon-based nanomaterials, metallic nanoparticles, and nanoballs have been introduced into electrode structure... 

Nakorn, P.N.A., 2008. Chitin Nanowhisker and chitosan nanoparticles in protein immobilization for biosensor applications. J. Metals, Mater. Minerals 18, 73—77. 

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