The Lanthanide Salts As Sensor Molecules Biology Essay

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The MSc project deals with the complexing of the lanthanide salts with the Phthalamide based ligands and testing of the Luminescence with the complex. Lanthanide complexes are most popular due to their f-f transitions, they have so many applications in the analytical chemistry and organic transformations, asymmetric synthesis and catalytic reactions and they have important significance in the biology. Structural change was taking place in the Phthalamide by hydrolysis with the NaoH. After this complex formed with this phthalamide based ligand. The Tb-Phthalamide complex was tested for the phosphorescence. Tb-Phthalamide complex absorbs the energy at UV-Visible region and goes back to ground state by emitting photon. It was observed that when the absorption goes down the peaks will go down.

1.1 Luminescence:

Luminescence process involves the molecule absorbs energy from the Ultra Violet light or Infrared or visible light and the molecule goes in to the excited states and it will goes back in to ground state by emitting the energy. Luminescence is mainly divided in to two types. They are Fluorescence and Phosphorescence. The below figure is showing the how molecule absorbs and goes into to the excited state and it also explains the fluorescence and phosphorescence techniques.

Figure 1 is showing the luminescence and fluorescence and phosphorescence techniques.(ANON, 2006)

Fluorescence technique: Fluorescence involves the same process as the molecule absorbs energy and goes in to excited state and back in to the ground state by photon emission. The below figure is showing the fluorescence technique.

The figure 2 is explaining the fluorescence technique.

The fluorescence involves the singlet states. The excited state has a life span of 10-9 to 10-7. If there is any change in the photon energy that will leads to the shifting of the fluorescence spectrum and it will leads to the longer wavelength. This phenomenon is called the Stokes Shift. The reaching of the molecule in to the zeroth level or the ground state by emitting photon is depends up on the energy separation between the singlet excited states. Sometimes the molecules will undergo Internal Conversion this is a Radiation less process. In this process the molecule will absorbs energy and it will come down in to ground state without emitting the photon. Instead of emitting the photon all the excitational energy will converts into heat. There is no vibrational relaxation of the molecule. This process is not an efficient one. The Internal Conversion process will occur some times in some Aromatic hydro carbons. The fluorescence technique involves shorter excitation states. The quenching of the fluorescence will occurs due to some effects like pH, Heavy atoms, Temperature, Oxygen atoms. Not all of the molecules will occur fluoresce, fluorescence technique is used for the sensitive detection [1].

Applications of the fluorescence technique: Fluorescence technique becomes an efficient tool in Analytical Chemistry and Biological systems. Researchers were done lots of the researches in the fluorescence techniques. Discovery of the Metal enhanced fluorescence (MEF) leads to the systemic changes in the fluorescence. The metal fluorophores will increases the photo stability and fluorophore emission intensity. Some of the applications of the MEF involve the Amplified directional emitting, enhanced wavelength radiometric emitting, and amplified immune assay detection [2], Research work is going on from since thirty years on the Fluorescence microscopy in Coal petrology. Now also improvement works are going on to improve the performance of the technique. Present improvement is going on in improving the light source for fluorescence excitation [3]. Florescence technique is very popular mechanism in detection. Recent developments have given possibility for fluorescence based sensing. One is the MEF which was already discussed and another one was use of the semiconductor non crystals which is called Quantum Dots (QD). The quantum dots technique is more efficient as comparing to the Metal enhanced fluorescence. Quantum dots will give high photo stability and broad excitation spectrum and narrow emission band [4]. Tryptophan like fluorescence was appropriate for the detection of the waste water pollution. This is also helpful to find out the quality of the water [5]. Fluorescence imaging is using to find out the oral mucosal cancers. Fluorescence photography was very much useful for the adjacent diagnostic method to differentiate the oral cancer. Some questions has been raised due to the occasional false positive reports and skin photo sensitivity. The progress of the auto fluorescence will increases with the increase of the surface area where the constituents are released. Auto fluorescence will decreases with the decrease of the area and it will leads to decrease of the accumulation.[6]. Fluorescence measurements will give high detection sensitivity, and it can be performed in any type of solution. Fluorescence basic analogues are also useful in the DNA protein interactions and in the base pairing of the analogues. Fluorescence techniques were also helpful in studying the structure and dynamics of the RNA and DNA [7]. The fluorescence fluctuation spectroscopy (FFS) was also the one of the powerful technique which was using in the biological research. This gives the valuable information about the Diffusion Coeffcients, Concentration, Reaction rates. There are two types of techniques in the FFS. They are Fluorescence Correlation Spectroscopy (FCS), Dual-color fluorescence cross-correlation spectroscopy (DC-FCCS). This technique fits to any size of the molecule but some problems involve in the applications they are the problems in labelling the macromolecules and their uneven distribution of the labels and possibility of the quenching is creating the problems. Apart from these problems DC-FCCS is appropriate for studying the ligand-dependent interactions of the androgen receptor. This method allows to find out the concentration of the free molecules and as well as the bounded molecules. DC-FCCS allows finding out any size of the molecules [8].

Phosphorescence technique: phosphorescence technique involves the absorbing the energy directly from the ground state, phosphorescence involves triplet states. This process is a spin-dependent internal conversion process. Generally the internal conversion process will occurs at the 10-8 sec of the life time. Vibrational coupling between the singlet state and the triplet state involves in the mechanism of the internal conversion process. Singlet-triplet states are less likely occurring than the singlet-singlet states. Some internal conversions will occur at the 10-13 sec. It is a radiation less process and after completing this molecule will go in to the zeroth level of the triplet state. The triplet state life time is much longer than the singlet state. So the excitation state will enhances. The radio transition state will occurs between the lowest triplet state and the ground state. Then the photon will emit in this process. The occurrence of this emission is called the phosphorescence [1].

Applications of the phosphorescence technique: By using the Solid-Surface room temperature phosphorimetry (SSRTP) we can find out the areas of the compounds this is the one of the analytical technique. The sample procedure is very simple in the low temperatures of the phosphorimetry and the room temperature phosphorescence will have some advantages like Stokes shift and it is easy to reduce the background scattered light. SSRTP technique is same like Electron immune assay (EIA). So this technique can used for the micro determination and it can be helpful to determine the phosphorimetry directly after immune system. And there is no need to use the special equipment in this technique. The combination of the substrate and the heavy atoms will enhance the phosphorescence signal [9,10]. The phosphorescence technique is widely using in the chemical sensing. The phosphorescence optical probes are more advantageous as comparing to the optical probes of the fluorescence technique. This is because due to the longer life times of the triplet states and due to the wide separation of the excitation and the emission peaks. The low phosphorescence levels were specially used for the immobilization of the chemical indicators. The immobilization of the chemical indicators will helps to rise the optical sensors with high signal to noise ratio's [11]. The finding of the O2 by using phosphorescence of the pd(II)-meso-tarta-meso-tarta-(4-tetrabenzoporphyrin) and it was first introduced by the Wilson and his colleagues. The phosphorescence was inversely related to the dissolved oxygen [12]. The Trp phosphorescence of the proteins in silica hydrogels will gives the delayed emission for examining the potential protein structure dynamics by using that silica matrix [13].

1.2 Lanthanides: Lanthanides are the well known as the Rare Earth Elements (REE). REE's are the 4f electrons they are from 57-71 and they are in the inner transition elements of the periodic table. REE's are present in the 1/4th of the metal content of the earth's crust. They are present in the two types of geometrical environments one is alkaline rocks and carbonalites and the second one is beach and inland placers. Country wise split of REE's are like this China (80%), USA (10.89%), India (4.93%), Russia (1.11%), South Africa (0.90%), Canada (0.4%), Australia (0.4%), others (1.36%). Usages of the REE's in different industrial sectors were like this Catalysis/chemicals (39%), Metallurgy (30%), Glass/ceramics (25%), and Phosphors/Electronics (6%). It is very difficult to find out the lanthanides when they are present together [14]. One of the more stable oxidation state of lanthanides is +3. Other two oxidation states are +2 and +4 but they will come back in to +3 oxidation state. Some rare earth elements lose electrons to become +3 for eg- Sm2+ , Eu2+, Yb2+, and they are the good reducing agents, and some elements will gain electron to become +3 oxidation state these are some elements which will gain electrons in aqueous solution to go to +3 oxidation state they are Ce4+, Pr4+,Tb4+. Lanthanide ions are highly charged +3 size of the lanthanide ions are very large they will yield small charge to size ratio with low charge density. So they are having the poor tendency to form complexes. Lanthanide complexes will form complexes with the strong chelating agents like EDTA, Oxine etc [15]. . Increase of the lanthanide luminescence will occur with the help of Quantum dyes as labels [16]. Commonly the solvent extraction is using now a day to separate the lanthanides [17]. Lanthanide luminescence is so much using now a days and the usage of the Eu and Tb also increased so much due to the direct laser excitation techniques [18]. Lanthanides have the below applications in the Biological effects they are Hormone like activity, Promoting the growth, Bacteriostatic effect, Influence on lipid peroxidation, Inducing apoptosis, Stabilizing and destabilizing the cytoskeleton [19]. Coordination numbers of the lanthanides was developed in 1960's. Some factors will affect the coordination number. Lanthanides with the large sizes are guessing the high coordination numbers. Some examples of the lanthanide complexes coordination numbers are given below [20].

Table 1:


Lanthanide Complexes

Coordination number


[La(edta)(OH2)3]-5 H2O














Table 1 is showing some examples of coordination numbers of Lanthanide complexes.

1.3 Lanthanide Luminescence: The lanthanide complexes are mainly noted for their f-f transitions due to these f-f transitions the emission bands are highly and narrow. Due to this special reason the lanthanide ions are using for different types of biological and chemical applications. And the europium and the terbium ions are of special interest because of their particular spectroscopic effects such as large strokes and shifts. The lanthanide ion excited states is very popular due to the energy transfer from the ligand or ligands. The lanthanides will produce an well defined energy levels. In 1990 the quenching of the excited states of the lanthanide ions was published. The ligands will serve as sensitizers. The energy transfer from ligand to lanthanide ions will occur. This is called the Antenna effect. The basic excited states by lanthanide salts and they will chelates in solution and it in the mechanism of the Antenna effect involves the energy transfer between luminescent ions and the ligands. The ligands have different roles. of the one of the ligands is to collect the photons provided by light source to allow energy transfer. And in the other hand it will helps as a shield against solvent to avoid non radioactive deactivation process. The energy transfer from the ligand to Ln(III) ion occurs from the lowest triplet state energy travel (T1) of the ligand to the resonance level of the Ln(III). This energy transfer is the one of the most important process that influences the luminescence properties. The selective coordinating capacity and hard binding sites will acquire with the help of the podand type ligand. In all of the podand type ligands the amide types are most important and they will produce a strong luminescence.[21,22,23].

Figure 3:

Figure 3 is explaining about the antenna effect and here the excitation will be absorbed first by a organic chromophore and then it will transfer it to the metal.

The Raymond group was used two organic chelates they are the 2-hydroxy isophthalamide and another one is the 1-hydroxypyridin-2-one'1,2-HOPO chelate. These two are anionic in nature and oxygen donors and they will act as an antenna for absorbing incident light. They contains the differing in back bones such as alkyl and aryl amines they will control the density of the ligand [24].

Figure 4 is showing the ligands which was used by the Raymond group.(Ref: Ken Reymond)

1.4 Applications of the Lanthanide Luminescence: So much development was occurred in the recent years in the lanthanides. Lanthanides were uses for the organic transformations, asymmetric synthesis and catalytic reactions and they have important uses in the organic chemistry. The chemistry of the lanthanide has important significance in the biology. All types of lanthanide complexes are so many useful but specially macrolytic lanthanide complexes are very useful due to their desirable thermodynamic and kinetic stabilities indeed helpful to study the physico chemical properties in solution.[25]. Lanthanide chelates shows special interest in the molecular multi photon absorption. Molecular multiphoton absorption is an simultaneous absorption of several photons and it was predicted by M. Goppert-Mayer in 1931. But this process requires powerful excitation sources like lasers. The recent developments in the lanthanide luminescent bioprobes will allow us to find out the important properties like water solubility, cell permeability, non cytotoxicity, kinetic inertness, thermodynamic stability at physiological stability. Lanthanide luminescent bioprobes are thermodynamically stable, kinetically inert and they will display a good sizable luminescence [26].

Figure 5:

The figure 5 is showing the podand type ligand (Ref: Tang.K).

Luminescent trivalent lanthanide ions are using so much due to the potential applications. The main compounds using in the lanthanide luminescence were Eu3+, Tb3+, Dy3+, and Sm3+ complexes. These all complexes will show the emission in the visible region. But mainly the chelates of the Eu3+, Tb3+, has a great interest because they are the luminescent markers in fluoroimmuno assay. Currently scientists showing good interest in the infrared luminescence of lanthanide complexes. They have good applications in emitting diodes , tele communication, optical fibre, light amplifier, laser etc. new chromophores are developed because the f-f transitions are forbidden the absorption coefficients are normally very small and the emission rates are normally very low. And sometimes the quenching vibrational deactivation will occurs. High energy oscillations such as O-H bonds and C-H bonds of the aromatic rings can able to quench the excited states and it will leads to decreased luminescence intensities and shorter excited state lifetimes.[27]. The photoluminescent europium complexes was first identified by Weissman in 1942. Now organic electroluminescence was studying because in this it is able to study especially at large flat panel display. And organic electroluminescence will give great results by improving the bright ness and electroluminescent efficiency. Luminescent bio assays was developed using lanthanide luminescent it is time resolved. The lanthanide complexes were used as a emissive optical probes. These probes can produce long life times and the sharp emission brands. Lanthanide complexes have an very good attractive properties as optical probes. The lanthanide complexes will give long emission life times. So many works will deals with the emissive complexes of the Eu(III) and Tb(III) because the excited states of the ions are less sensitive to vibrational quenching by energy transfer to OH ,NH or CH oscillations. [28]. Chemistry of lanthanide coordination is one of active research field in inorganic chemistry.[29].

1.5 Nature of the Project : All lanthanide salts are f block elements corresponding to the filling of electron shell, filling of the 4f electron shell. Emission from the lanthanide cations are generally weak due to electronic transitions are forbidden. The emission of the lanthanide salts is strong enough by suitable coupling between the electronic states of organo-chromophores and in those earth ions the Ln3+ the ligand to metal is effective energy transfer is more effective. Lanthanide cations emitting in the visible region can be efficiently sensitized with the coupling of the ligands[30,31] Phthalamides, phthalamates and 2-hydroxyisophthalamide group are very good ligands for coupling with the Ln3+ cations. Ligand to metal transfer will occur at this stage. And this energy transfer stage is called the "antenna effect"[30]. Phthalamide based ligand was prepared with the two different amines. One is diethyl amine and another one is Tris-(2-amino ethyl) amine. The reactions was given below. After completing the preparation the recrystallization process was done to purify the compound hydrolysis was done with the 0.2% NaOH to open the ring for coupling with the lanthanide complexes (europium and the terbium ions).[15,32] The synthesis of the phthalamide based ligand with two different amines were given below.

Scheme 1:

Figure 5: Figure 5 is showing the synthesis of the compound 1

Scheme 2:


Phthalic anhydride Tris- (2-amino ethyl) amine

Reflux with acetic acid for 3 hours


Figure 5 The synthesis of the compound 2