A facile cost-effective electrolyte-assisted approach and comparative study towards the Greener synthesis of silica nanoparticles

Abstract

Nowadays, silica nanoparticles are gaining tremendous importance because of their wide applications across different domains such as drug delivery, chromatography, biosensors, and chemosensors. The synthesis of silica nanoparticles generally requires a high percentage composition of organic solvent in an alkali medium. The eco-friendly synthesis of silica nanoparticles in bulk amounts can help save the environment and is cost-effective. Herein, efforts have been made to minimize the concentration of organic solvents used during synthesis via the addition of a low concentration of electrolytes, e.g., NaCl. The effects of electrolytes and solvent concentrations on nucleation kinetics, particle growth, and particle size were investigated. Ethanol was used as a solvent in various concentrations, ranging from 60% to 30%, and to optimize and validate the reaction conditions, isopropanol and methanol were also utilized as solvents. The concentration of aqua-soluble silica was determined using the molybdate assay to establish reaction kinetics, and this approach was also utilized to quantify the relative concentration changes in particles throughout the synthesis. The prime feature of the synthesis is the reduction in organic solvent usage by up to 50% using 68 mM NaCl. The surface zeta potential was reduced after the addition of an electrolyte, which made the condensation process faster and helped reaching the critical aggregation concentration in a shorter time. The effect of temperature was also monitored, and we obtained homogeneous and uniform nanoparticles by increasing the temperature. We found that it is possible to tune the size of the nanoparticles by changing the concentration of electrolytes and the temperature of the reaction using an eco-friendly approach. The overall cost of the synthesis can also be reduced by ∼35% by adding electrolytes.

Scheme 1. Schematic representation of silica nanoparticle synthesis using an electrolyte.

Fig. 1. Characterization of silica nanoparticles (a) TEM analysis of SiNPs synthesized using optimized electrolytic concentrations. (b) FTIR spectra of SiNPs synthesized using electrolytic and non-electrolytic pathways in different compositions of ethanol. (c) XRD patterns of SiNPs synthesized using electrolyte and non-electrolyte-assisted pathways.

Fig. 2. (a) Changes in the relative concentration of silica nanoparticles with time. Varying the ethanol concentration from 60% to 30%. (b) Comparing the electrolytic and non-electrolytic processes in 30% and 40% ethanol.

Fig. 3. (a) Particle size distribution (PSD) of different alcoholic concentration systems: 30% and 40% alcohol with electrolyte and without electrolyte; 50%, and 60% alcohol non-electrolyte systems. (b) Histogram of the particle size distribution (PSD) obtained from a TEM study of synthesized particles using 50% ethanol. (c) TEM image of particles synthesized using 67 mM electrolyte and 30% ethanol. (d) Histogram of the particle size distribution (PSD) obtained from a TEM study of particles synthesized using 67 mM electrolyte and 30% ethanol.

Fig. 4. Size distribution of particles while maintaining 67 mM electrolyte concentration in 30% ethanolic solvent.

Fig. 5. (a) The concentration changes of silica nanoparticles between the electrolyte-ethanol and electrolyte-isopropanol (IPA) systems. (b) PSD of silica nanoparticles synthesized in 30% methanol, ethanol, and isopropanol with the electrolyte system.

Fig. 6. (a) ln(C₀/C_t) vs. time plot for the kinetics study of the nucleation process at 18 °C, 28 °C and 38 °C for 30% alcohol with electrolyte. (b) PDI of silica nanoparticles, synthesized in a 60% ethanol-non-electrolytic system. The temperature variation is 18 °C, 28 °C, and 38 °C. The PDI values decrease with increasing temperature, indicating a narrow size distribution (Fig. S4 in ESI†).