Corrugated and nanoporous silica microspheres: synthesis by controlled etching, and improving their chemical adsorption and application in biosensing

Abstract

Corrugated and nanoporous silica microspheres were synthesized by simple controlled etching of smooth spherical silica microspheres of various sizes with aqueous KCN or KOH solution. The smooth silica microspheres were prepared by the Stöber method. The structure and morphology of the etched microspheres were controlled by varying the type and concentration (pH value) of the etchants and the etching time. Upon etching the original silica microspheres with a higher concentration of etchant for a longer time, highly corrugated and hollow silica microspheres were obtained. Shorter etching time and a lower concentration of etchant have resulted in only moderately corrugated microspheres. Comparative studies revealed that the etching with KCN solution proceeds more slowly due to the milder concentration of OH⁻ ions it generates compared to a similar concentration of KOH solution. By optimizing the etching process, silica microspheres with increased adsorption capacity for chemicals such as rhodamine 6G can be obtained. By encapsulating gold nanoparticles and horseradish peroxidase into the etched microspheres having optimized corrugated structures and then casting the resulting nanocomposite materials on a glassy carbon electrode, sensitive electrochemical biosensors for the detection of micromolar concentrations of H₂O₂ were fabricated. The correlations between the microspheres' size, the etchant's concentration and the etching time with the structures and shapes of the etched microspheres, their adsorption properties to chemicals, and their potential in biosensing application were established. The synthetic method also proved to be applicable in producing corrugated/hollow titania microspheres. This controlled etching synthetic method to produce corrugated metal oxide microspheres is versatile and is useful for controlling nanomaterials' structure and morphology, enhancing their surface area and adsorption capacity of chemicals and drug molecules, as well as making sensitive electrochemical biosensors.