Thiols are organic compounds that contain the functional group composed of a sulphur and a hydrogen atom (-SH). Alkylthiols are also referred to as Sulfhydryls or Mercaptans. The thiol functional group is the sulphur analogue of the hydroxyl group. Hence, the synthesis of thiols is analogous to the method of making alcohols. Thiols are formed when a halogenoalkane is heated with a solution of sodium hydrosulphide.
CH3CH2Br + NaSH heated in ethanol (aq) → CH3CH2SH + NaBr (2.12)
Sulphur and oxygen are both in group VI of the periodic table. Sulphur is in period 3, whereas, oxygen is period 2. Therefore, thiols and alcohols share some of their chemical bonding properties. Sulphur anions are better nucleophiles than oxygen atoms as the sulphur nucleus exerts a lesser force on its electrons, hence, reactions to form thiols are quicker than those to form an alcohol and have a higher yield. The difference of electronegativity between sulphur and hydrogen is fairly low. This gives thiols a low dipole moment and makes them almost non-polar.
The advantage of using thiols as gas sensitive films is the flexibility to control their properties through molecular level design. This allows for the inclusion of a polar group to the otherwise non-polar thiol polymer. Thiols also bond readily with metal particles and studies of thiols bonding with gold particles have been carried out by Brust et al (Brust et al., 1998; Brust et al., 1995; Brust et al., 1994). The flexibility of thiols to allow polar and non-polar groups while acting as linker structures with gold particles, which provided an effective conductive channel in the films, was the reason derivatives of gold alkyl-thiols were used as the sensor film in this study.
2.4.1. Gold nanoparticlePolymer composite films
The physical and chemical properties and thus the selectivity of the polymer films can be modified by forming composite materials with metal nanoparticle cores. The nanoparticles provide signal transduction and hence, enhance the conductivity of these materials, which otherwise may have a very high resistivity. This section focuses on the choice of colloidal gold as the metal nanoparticle material. Gold particles were chosen for their well established chemistry with organic ligands containing the thiol group (Alvarez et al. 1997) .
Figure 2.7 Examples of thiol molecules commonly used with functionalised
gold surfaces. From left to right: (a) Mercaptoundecanoic acid, (b)
Mercaptoacetic acid, (c) Cysteamine, (d) 4mercaptoaniline, (e) 4
mercaptobenzoic acid and (f) 4mercaptoanisole (TheryMerland et al. 2006)
The growth of these crystals depends on the activation energy required and the availability of this activation force. Alvarez et al. show that the growth of gold nanoparticle clusters can be easily controlled and slowed down and singularly stable structures can be brought to a halt. The activation energy can be removed by lowering the temperature or by a weakly binding passivating agent that also acts as a mild etchant in an otherwise inert environment. Removal of this energy results in slowing down or halting the colloidal growth process, thus allowing control over the particle size. These gold clusters grow from atomic level dispersion and act as thermochemical ‘sinks’ as they accumulate. The weakly bound surface passivating monolayer has little or no effect on the stability of these particles which can be obtained as the exclusive product of the process.
Monodisperse colloids of CdS and CdSe have been grown by Murray et al. (Murray et al., 1993) and the kinetics of the growth processes have been worked on by Reiss (Reiss 1951). Gold nanocrystal growth over a range of 1.5-20 nm diameter have
been grown in a controlled surfactant concentration environment, i.e. by varying the gold to thiol ratio by Leff and co-workers (Leff et al., et al. 1995).
Gold clusters and its colloidal and compound forms are the best understood metallic systems. Gold is also mostly chemically inert and possesses low surface energy, which gives small gold crystallites extensive stability as compared to bulk gold. On a per-surface-atom basis gold’s surface energy is about a tenth of that of the bulk cohesive energy (3.9 eV), 90% of which may be retained by gold clusters of as little as 75 atoms. Usually this is only 60% at aggregation. This property of gold allows using a weakly binding group such as alkylthiolates (SR) for protecting the nanometer scale gold surfaces of large clusters. This may be represented as Au-SR. Previous studies have shown n-Alkylthiolates (SRn) to form protective compact, ordered monolayers where thiolates (-SR) or dialkydisulfides (RSSR) reversibly attach to various gold surfaces (Dubois et al., 1992). This behaviour is down to the gold surface atoms interacting with the non-bonding ‘s’ orbital of an intact RSSR molecule (Fenter et al., 1994). This property can be observed amongst larger Aun(SR)m clusters, where ‘n’ is much larger than ‘m’, and hence the molecule appears flat to the RSSR unit as in extended surface systems.
Thiols are well-known to form stable, self-assembled monolayers on gold surface (Camillone et al., 1991; Fenter et al., 1993; Laibinis et al., 1991). With the above mentioned properties of thiol as gas sensitive materials and the way gold particles behave in their presence, gold nanoparticle composite polymers based on this linker functional group were chosen for the purpose of this study.