Studying nanomaterials based receptors for molecular recognition by using electronic and vibrational spectroscopy

Abstract

Nanomaterials have gained attention recently because of their wide range applications in various fields of physical, chemical, material and biological sciences due to their unique chemical, thermal, electrical, mechanical and optical properties. Nanomaterials can be classified into three categories which is based on the materials used for fabrication named as organic nanomaterials, inorganic nanomaterials and hybrid nanomaterials. In the present work, our focus would be on organic and hybrid nanomaterials. In organic nanomaterials, we have worked on organic nanoparticles and carbon quantum dots whereas in case of hybrid nanomaterials our study was focused on gold – organic nanohybrids and their potential in sensing applications. In order to do so we have employed UV – visible absorption and fluorescence spectroscopic tools for studying the bulk activities in the solution phase. In order to extract detailed structural and molecular level insights about the nanomaterials and their activities in the solution phase, we have utilized sum frequency generation (SFG) spectroscopy. SFG spectroscopy is based on second order nonlinear optical process which is intrinsically sensitive to probe the interfacial activities. It provides the vibrational spectrum of the molecule solely residing at the interfacial region. The organic – inorganic nanohybrid was fabricated using gold nanoparticles and organic receptors. The gold nanoparticles surface plasmon resonance (SPR) and fluorescence quenching properties make them suitable for the applications in recognition of analytes. We have decorated gold nanoparticles with calixarene and naphthalimides derivatives for the sensing of analytes. Calixarene coated AuNPs were found to be quite selective for the colorimetric recognition of iodide ions due to change in the SPR upon aggregation. The naphthalimides coated AuNPs were used for the detection of anions (cyanide and iodide ions) and okadaic acid which is a marine toxin and causes diarrhetic shellfish poisoning. Carbon quantum dots (CQDs) have been extensively explored in various fields of science after their accidental discovery in 2004 by Xu et al. while purification of single wall carbon nanotubes. In comparison to conventional quantum dots, CQDs have better water solubility, high quantum yield, biocompatibility, high photostability and low toxicity. CQDs are comprehensively being used in the arena of chemical sensing, photocatalysis and bioimaging. The CQDs were functionalized with ionic liquids (IL@CQDs) and they used as catalyst the rearrangement reaction of the active ester intermediate of 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) amide coupling reaction. With this aim the two different ionic liquids were embellished on the surface CQDs and the catalytic activity was monitored with and without the ionic liquids. The product formation was confirmed with 1H NMR, 13C NMR and single crystal XRD. Organic nanoparticles (ONPs) of the rearranged product were fabricated using re-precipitation method and their anion binding studies were carried out using fluorescence spectroscopy. It was observed that the ONPs gave enhancement in the fluorescence profile specifically with highly toxic cyanide ion. The selective binding was the result of restricted rotation of the molecule around the single bond due to hydrogen bond formation of the cyanide with the –NH groups of the product. The sensing of CN- ions was shows more than 90% accuracy in real sample analysis and quite low detection limit. The drugs usage has been increasing day by day thus causing excessive drug abuse and depression is one the major outcome of this situation. In this section, we have used CQDs which were synthesized from citric acid with terbium for the detection of antidepressant drug clomipramine. The detection of the drug has been carried out in the aqueous medium using fluorescence spectroscopy. The selective binding was occurred due to aggregation induced enhancement of fluorescence intensity. The molecular level interactions of the CQDs-Tb were carried out using vibrational sum frequency generation spectroscopy. By using conventional spectroscopic tools, it is difficult to probe the interfacial activities and the molecular structure of the CQDs and their impact on water structure. Therefore, we have used a highly surface sensitive and chemically specific sum frequency generation (SFG) spectroscopic tool to probe the CQDs and pristine water interface. It was found that the presence of the CQDs strongly affect the water ordering and its hydrogen bonding coordination at the air-water interface. The enhancement in the intensity of peaks –OH stretching region clearly depicts CQDs role in changing the ordering of the interfacial water structure. The CQDs switched ON the water SFG signal however upon binding with highly toxic mercuric ions the SFG signal was switched OFF. The process was found to be reversible. As soon as the mercuric ions were engaged with iodide ions, the water signal was switched ON again. The same results were then validated from the surface tension and contact angle measurements inferring that one can alter the physical properties of the solution.