Saurabh Ahalawat

Sol-Gel processing of silica nanoparticles and their applications.

Recently, silica nanoparticles (SNPs) have drawn widespread attention due to their applications in many emerging areas because of their tailorable morphology. During the last decade, remarkable efforts have been made on the investigations for novel processing methodologies to prepare SNPs, resulting in better control of the size, shape, porosity and significant improvements in the physio-chemical properties. A number of techniques available for preparing SNPs namely, flame spray pyrolysis, chemical vapour deposition, micro-emulsion, ball milling, sol-gel etc. have resulted, a number of publications. Among these, preparation by sol-gel has been the focus of research as the synthesis is straightforward, scalable and controllable. Therefore, this review focuses on the recent progress in the field of synthesis of SNPs exhibiting ordered mesoporous structure, their distribution pattern, morphological attributes and applications. The mesoporous silica nanoparticles (MSNPs) with good dispersion, varying morphology, narrow size distribution and homogeneous porous structure have been successfully prepared using organic and inorganic templates. The soft template assisted synthesis using surfactants for obtaining desirable shapes, pores, morphology and mechanisms proposed has been reviewed. Apart from single template, double and mixed surfactants, electrolytes, polymers etc. as templates have also been intensively discussed. The influence of reaction conditions such as temperature, pH, concentration of reagents, drying techniques, solvents, precursor, aging time etc. have also been deliberated. These MSNPs are suitable for a variety of applications viz., in the drug delivery systems, high performance liquid chromatography (HPLC), biosensors, cosmetics as well as construction materials. The applications of these SNPs have also been briefly summarized.

Preparation of Silica Nanoparticles and its Beneficial Role in Cementitious Materials

Spherical silica nanoparticles (n-SiO2) with controllable size have been synthesized using tetraethoxysilane as starting material and ethanol as solvent by sol-gel method. Morphology and size of the particles was controlled through surfactants. Sorbitan monolaurate, sorbitain monopalmitate and sorbitain monostearate produced silica nanoparticles of varying sizes (80-150 nm), indicating the effect of chain length of the surfactant. Increase in chain length of non-ionic surfactant resulted in decreasing particle size of silica nanoparticles. Further, the size of silica particles was also controlled using NH3 as base catalyst. These silica nanoparticles were incorporated into cement paste and their role in accelerating the cementitious reactions was investigated. Addition of silica nanoparticles into cement paste improved the microstructure of the paste and calcium leaching is significantly reduced as n-SiO2 reacts with calcium hydroxide and form additional calcium- silicate-hydrate (C-S-H) gel. It was found that calcium hydroxide content in silica nanoparticles incorporated cement paste reduced ~89% at 1 day and up to ~60% at 28 days of hydration process. Synthesized silica particles and cement paste samples were characterized using scanning electron microscopy (SEM), powder X-ray diffraction (XRD), infrared spectroscopy (IR) and thermogravimetric analysis (TGA).

Reduction of calcium leaching in cement hydration process using nanomaterials

Abstract Continuous efforts are being made to understand the mineralogical and morphological properties of cement hydration at the early stages for speedy construction. Use of nanoparticles may provide faster kinetics due to their high surface/volume ratio. Spherical dispersed silica nanoparticles were prepared by employing the sol–gel method. Scanning electron microscope (SEM) and X-ray diffraction (XRD) studies revealed that the prepared nanoparticles are amorphous and uniform in size (∼50 nm). Further, these particles were incorporated into cement paste and investigated through SEM, XRD and thermogravimetric analysis techniques. Incorporation of silica nanoparticles into cement paste improved the microstructure of the paste, and calcium leaching is significantly reduced as silica nanoparticles react with calcium hydroxide (CH) and probably form an additional calcium–silicate–hydrate (C–S–H) gel. It was found that the CH content in the silica nanoparticle incorporated cement paste reduced by ∼86% at 1 day and up to ∼62% at 28 days of the hydration process. The XRD studies revealed that the addition of 2·5% (w/w) silica nanoparticle powder was found to be optimal to significantly digest the CH. Morphological features as studied through SEM showed that the microstructure of silica nanoparticle incorporated cement paste becomes significantly denser, and a more compact C–S–H gel is formed, thereby improving the mechanical properties of the resulting cement paste. Insight and tuning of cement based materials at nanolevel may thus greatly help in achieving ultrahigh performance concrete for durable and sustainable construction.

Granulometric synthesis and characterisation of dispersed nanosilica powder and its application in cementitious system

Abstract Dispersed, spherical particles of nanosilica with controllable size have been synthesised using a metal alkoxide, i.e. tetraethoxysilane, as starting material, ammonia as base catalyst and non-ionic surfactant as template by sol–gel method. Size of particles and dispersivity were controlled by varying the surfactant chain length and temperature conditions of the reaction mixture. Silica nanoparticles were synthesised using a series of non-ionic surfactants, namely Polyoxyethylene (20) sorbitan monolaurate (Tween 20) and Polyoxyethylene (80) sorbitan monooleate (Tween 80), at different reaction temperatures of 25, 50, 70 and 90°C. The particle size of silica nanoparticles gradually decreased with increasing carbon chain length of the surfactant and at higher temperature particle size became larger. Furthermore, these silica nanoparticles are incorporated into the cementitious system to improve the mechanical properties and reduce calcium leaching in the hydration process. Addition of silica nanoparticles into cement paste improves the microstructure of the paste, and calcium leaching is significantly reduced as silica nanoparticles react with calcium hydroxide, thereby forming a denser calcium–silicate–hydrate gel structure. Synthesised silica nanoparticles and microstructure of cement paste incorporated with silica nanoparticles were analysed using scanning electron microscopy, powder X-ray diffraction, infrared spectroscopy (IR), 29Si MAS NMR and thermogravimetry analysis for morphological and mineralogical attributes.

Determination of E. coli by a Graphene Oxide-Modified Quartz Crystal Microbalance

ABSTRACT A biosensor for the determination of Escherichia coli using graphene oxide on the crystal (gold) surface was fabricated by the drop cast method. The E. coli sensing characteristics of the biosensor, such as a change in frequency, were examined by exposing the graphene oxide-coated crystal to various functionalization steps at room temperature. Graphene oxide was functionalized by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride–N-hydroxysuccinimide to covalently conjugate β-galactosidase antibodies to recognize microorganisms that produce this material. Frequency changes in the quartz crystal microbalance are dependent on the absorbed/desorbed masses of the analytes on the functional surface of the crystal. In addition, various characterization techniques were optimized for the morphological elemental analysis of the nanocoating that included field emission scanning electron microscopy, scanning electron microscopy, and electron diffraction spectroscopy. This surface was used in a quartz crystal microbalance nanoplatform for the rapid, sensitive, and label-free detection of E. coli. Under optimal conditions, the frequency of quartz crystal microbalance biosensor was directly proportional to the concentration of antigen with a dynamic range from 0.5 mg mL−1 to 5 ng mL−1 and a minimum detection limit of 5 ng mL−1, and a sensitivity of 0.037 Hz g ml−1 cm−1. These results show that the graphene oxide-coated crystal had excellent performance for E. coli. This research reports a simple, inexpensive, and effective highly stable biosensor using graphene oxide as the sensing medium.