Lanthanide Based Hydrogels in Sensing, Energy Transfer and Nanoparticle Synthesis
Chapter 1: Luminescence property of lanthanide and its applications Lanthanides are well-known for their unique luminescence property and have found widespread applications in sensing, bioimaging, lasers, optoelectronic devices, etc. Due to Laporte forbidden f-f transitions, lanthanides have very low intrinsic emission. The problem can be overcome by use of an ‘antenna’, which is an organic chromophore with excited state energy higher than the lanthanides’ emitting levels. Thereby it is possible to get highly emitting lanthanide complexes through energy transfer from the ‘antenna’. Due to long lifetime of lanthanides’ excited states, it's possible to perform time delayed measurement which is useful in bioassays and bioimaging since the short-lived background emission is effectively filtered. Research in supramolecular metallogels has grown rapidly in recent years, and already proven to have potential for designing advanced materials for a variety of applications, such as sensing, optoelectronics, catalysis, nanoparticle synthesis, biomedicine etc. A supramolecular gel where a lanthanide is an integrated part of it can combine the advantages of the supramolecular gel along with the unique property of lanthanide luminescence and thus such materials can be explored for potential applications. This chapter discusses the background information on the unique luminescence of lanthanides, and some examples of the applications of lanthanide complexes and lanthanide based gels. Chapter 2: Lanthanide luminescence based enzyme sensing in hydrogels This chapter describes the use of Tb/Eu luminescence in the sensing of biologically important enzymes. We discovered the sensitization of Eu(III) in Eu-cholate gel by 1-hydroxypyrene, and of Tb(III) in Tb-cholate gel by 2,3-dihydroxynaphthalene. These two sensitizers were covalently modified and sensitizer-appended hybrid (artificial) enzyme substrates were prepared for a few biologically important hydrolases. The covalently modified sensitizer termed as “pro-sensitizers”, didn't sensitize Tb(III)/Eu(III) in the hydrogel and no photoluminescence was observed. In the presence of the appropriate enzyme in the hydrogel, the pro-sensitizer was cleaved to liberate the sensitizer, which led to an enhancement of luminescence with time. Alkaline phosphatase and β-lactamase were assayed using pyrene phosphate and pyrene-oxo-cephalosporanic acid derivatives, respectively, in Eu-cholate hydrogel (Figure 1). β-Galactosidase was assayed using Tb(III) luminescence in Tb-cholate gel. The enzyme detection was based on red/green luminescence response from the gel. To understand the behaviour of the enzymes in the hydrogel, kinetic parameters were determined. The detection of different enzymes was also demonstrated in natural/biological samples like blood serum, milk and almond extract. Figure 1. Three different pro-sensitizers used for alkaline phosphatase, β-lactamase and β-galactosidase assays Chapter 3: Enzyme sensing on paper discs using lanthanide luminescence Developing a user-friendly biosensor is of considerable importance in clinical and analytical chemistry. Paper based biosensor design is an emerging field of research and paper based point of care (PoC) testing devices have already found applications in clinical, veterinary, environmental, food safety, security etc. Paper is made out of natural cellulose fibres, and has advantages of low cost, biodegradability, biocompatibility, and user friendliness. Paper based sensors have been used for the detection of ions, glucose, proteins, nucleic acids, antigens etc., with mostly colorimetric, fluorescent, electrochemical, chemiluminescence and Electrochemiluminescence readouts. In this work, the non luminescent Tb(III) and Eu(III) were embedded on paper as their cholate hydrogels and were used for detecting different hydrolases. Pro-sensitizers, as reported in Chapter 2, were immobilized on paper for the detection of a specific enzyme. The “pro-sensitizer” released the sensitizer upon enzyme action and led to luminescence enhancement from the gel coated paper disc. By this way, four different hydrolase enzymes detection were carried out using Tb(III)/Eu(III) luminescence as the readout (Figure 2) and the practical utility was demonstrated by the detection of specific enzymes in natural/biological samples. This paper disc based enzyme sensing provides a simpler and user friendly approach over the contemporary approach of enzyme sensing typically carried out in solution. Figure 2. Paper based biosensors for hydrolase enzymes Chapter 4: Luminescence resonance energy transfer in self-assembled supramolecular hydrogels Luminescence resonance energy transfer is a phenomenon of energy transfer between a FRET (Förster resonance energy transfer) pair, where a lanthanide is the donor. Lanthanides have attracted attention for the last several decades for their unique luminescence properties. LRET is a FRET process along with added advantages of Lanthanides, i.e. long lifetime of the lanthanides and characteristics emission spectra. LRET has been used for studying interaction of biomacromolecues, immunoassay, bioassays, etc. LRET in either a supramolecular organogel or a hydrogel is still an unexplored field. In this work we showed the energy transfer from Tb(III) to two different red emitting dyes in Tb-cholate hydrogel (Figure 3). The self assembly processes during hydrogelation assisted the energy transfer process without any need for laborious synthesis. The energy transfer was confirmed by time delayed emission, excitation spectra and lifetime measurement in the hydrogels. Energy transfer was observed both in the gel and the xerogel states. These luminescent materials may find applications in optoelectronics. Figure 3. Energy transfer from DHN to Tb3+ and then to red emitting dyes (Rhodamine B & Sulforhodamine 101) in the Tb-Cholate hydrogel Chapter 5: Room temperature synthesis of Lanthanide phosphate nanoparticle using a gel as a soft template Lanthanide orthophosphates are an important class of rare earth compounds, and have widespread applications in laser materials, optical sensors, heat resistance materials, solar cell etc. There are several methods in the literature for the synthesis of rare earth phosphate nanoparticles. Most of these are based on hydrothermal, microwave assisted, micro emulsion, arrested precipitation etc., which invariably dependent on stringent conditions such as (i) high temperatures and pressures, (ii) inert atmosphere and (iii) the use of external capping agents as stabilizers. Synthesis of such nanoparticles under milder conditions would always be preferable. In this context, the preparation of nanoparticles using hydrogel as template can be a possible alternative approach. The LnPO4 nanoparticle synthesis was done by diffusion of Na3PO4 in Ln-cholate hydrogels. The particles were characterized by transmission electron microscopy (TEM) and powder XRD analysis. TEM showed the formation of 3-4 nm size particles with an ordered arrangement on the gel fibre. This work demonstrated that the lanthanide cholate gels have high potential for the synthesis, and immobilization of lanthanide phosphate nanoparticles at room temperature to produce new types of composite materials. (For structural formula pl see the abstract pdf file)
- Organic Chemistry (OC) 
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