Green Synthesis Of Functionalized Nanoparticles Using A Fractional Factorial Design: Impact On Particle Size And Distribution Optimization

With the escalating growth of nanotechnology and as the field has started to realize its potential, several conventions that involve nanoparticles with a wide scope of applications are being developed. Introduced to the public in 1959 by the physicist Richard Feynman in his popular talk “There is Plenty of Room at the Bottom,” this new concept was initially received as “obvious imagination” and went overlooked. As their name infers, nanomaterials are materials in which at least one dimension is ˂ 100 nm. When all three dimensions are ˂ 100 nm, the correct term is nanoparticles.

Possessing unique properties — including a manageable particle size (PS), a large surface area, a uniform particle size distribution (PSD), and a potential for functionalization with a variety of functional groups via facile synthetic approaches — silica (SiO₂) nanoparticles (SNPs) are now highly instigated in numerous areas of remediation applications. Therefore, the keyword in the quality of an application implicating SNPs would apparently depend on controlling the synthetic conditions in such a way that the yield would have a small PS and a consistent PSD.

In general, synthesis of nanomaterials can be grouped into two main categories: bottom-up and top-down methodologies. Yet, the bottom-up pathway is more economical, faster, and energy efficient and, thus, it is generally the route of choice for the synthesis of SNPs. Stöber synthesis — wet chemistry or sol-gel approach — is one of the most common methodologies adopted for the synthesis of SNPs with the looked-for properties. Several variables are known to influence such a reaction: concentrations of tetraethoxysilane (TEOS, the molecular precursor to produce SNPs), ammonium (a catalyst that boosts the hydrolysis and condensation reactions), water, and 3- Cyanopropyltriethoxysilane (CPTS), in addition to the reaction and stirring times.

Like any reaction, the quality of the yield is reliant on the control pursued over reaction variables. The traditional approach is to have a control over each individual reaction variable. In this approach, each variable is treated as an isolated territory and with the assumption that no variable-variable interactions exist. However, this is not the case in the majority of the reactions, and the information obtained following such an approach cannot be treated with a high degree of confidence.

A multivariate approach would be an ideal solution, then. In addition to the reduction in the number of experiments run, which in turn means a decrease in the amount of waste generated and an improved method greenness, a mathematical equation is obtained where the desired response can be estimated with any experimental setup, even without actually performing the experiment.

In the current endeavor and motivated by the importance of SNPs and our continuing efforts in the development of greener methods, synthesis of cyano-functionalized SNPs employing a fractional factorial design and multiple response optimization was reported. The target was to control the PS and obtain a uniform PSD following Stöber synthesis.

The investigation involved a screening of six factors and measurement of two responses, PS and PSD. The implemented design involved sixteen experiments in the base run being conducted in two blocks. The target was to minimize the PS as well as the PSD. Overlaid contour plots, as well as composite desirability function, were used to simultaneously consolidate both responses into a single performance characteristic.

Pareto chart indicated that the concentration of ammonia was the most affecting main variable. The interaction of TEOS*ammonia affected mainly PS, while PSD was influenced, along with TEOS*ammonia, by interactions of TEOS*Stirring time and TEOS*Reaction time. These conclusions were confirmed by an analysis of variance (ANOVA). Contour and surface plots were used to study the relationship between each of the two variables and the studied response in a 2-D and 3-D setup. Best factorial arrangements were obtained from the desirability plots.

Obtained SEM micrographs showed the homogeneity of prepared SNPs in general. Other characterization techniques confirmed the functionalization of the synthesized SNPs with the cyano-functional group.

These findings are described in the article entitled Application of fractional factorial design for green synthesis of cyano-modified silica nanoparticles: Chemometrics and multifarious response optimization, recently published in the journal Advanced Powder Technology. This work was conducted by Marwa S. Elazazy, Ahmed A. Issa, Maha Al-Mashreky, Maetha Al-Sulaiti, and Khalid Al-Saad from Department of Chemistry and Earth Sciences, College of Arts and Sciences, Qatar University.