AuNPs/colloidal stability

One year later, Brust and Schiffrin [28,29] published a method for AuNPs synthesis which has a considerable impact on the overall field in less than a decade because it allowed the facile synthesis of thermally stable and air-stable AuNPs of reduced dispersity and controlled size for the first time. In this method, the gold colloids are sterically stabilized by organic molecules having thiol, amide or acid groups in contrast to the citrate reduction method where the gold colloids are kinetically stabilized in aqueous solutions by an electrical double layer [28,29], The main advantage of the Brust method is that the gold particles behave in a way as chemical compounds. These AuNPs can be repeatedly isolated and... 

As mentioned in the previous section, the response, the stability and the enzyme activity found greatly enhanced at the MWCNT platform. Other than CNTs, AuNPs also possess some unique properties and recent years it has been widely employed in the biosensors to immobilize biomolecules. Thus in this section we discuss about the application of AuNP matrix for the immobilization of AChE for pesticide sensor development. With the use of AuNPs, the efficiency and the stability of the pesticide sensor gets greatly amplified. Moreover, the nanoparticles matrix offers much friendly environment for the immobilized enzyme and thus the catalytic activity of the enzyme got greatly amplified. Interestingly, Shulga et al. applied AChE immobilized colloidal AuNPs sensor for the nM determination of carbofuran, a CA pesticide [16], The enzyme-modified electrode sensor was also utilized for the sensitive electrochemical detection of thiocholine from the enzyme catalyzed hydrolysis of acetylthiocholine chloride (ATCl). The fabrication and the enzyme catalyzed reaction at the AuNPs coated electrode surface is shown in Fig. 6. 

The samples were modified in direct (glow, diode) Ar plasma on Balzers SCD 050 device under the conditions presented in 2.3.1. Immediately after the plasma treatment the samples were inserted into water solution (2 wt. %) of 2-mercaptoethanol (ME), methanol solution (5.10 mol/dm ) of biphenyl-4,4 -dithiol (BPD) or into water solution (2 wt.%) of cysteamine (CYST) for 24 hours. To coat the polymers with the gold nanoobjects the plasma treated pol)rmers with grafted thiols were immersed for 24 hours into freshly prepared colloidal citrate stabilized solution of Au nanoparticles (AuNPs), [44,46] or Au nanorods (AuNR), 0.1 mol/dm water solution of cetyltrimethylammonium bromide). 

At first, ICP analyses showed comparable Au concentrations (0.045-0.05 mg/1). The corresponding changes in the morphology, size distribution and stability of the prepared colloidal AuNPs were characterized by different methods as described below. 

In a first set of experiments colloidal AuNPs were obtained using ST as reductant and SC as stabilizer. UV-Vis spectroscopy is often used as quite sensitive technique for observing the formation of colloidal AuNPs because they display an intense absorption band in the region of 500-550 nm, especially when the particle size exceeds ca 5 nm [5]. 

Stability of colloidal AuNPs In order to stndy the inflnence of precursor on the stability of colloidal AuNPs, the Au ions were reduced in absence of stabilizer. The expected particle growth was measured by their zeta potential (Q, with the simultaneous intention to estimate their stability. The particles with zeta potential values >... 

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