Causes of Luminescence and Chemiluminescence

Causes of Luminescence and Chemiluminescence1.1. Luminescence PhenomenaLuminescence is the phenomenon of electric discharge of electromagnetic radiation in excess of thermal radiation. The radiation is usually in the apparent region of the electromagnetic spectrum, however, since the same basic processes may yield infrared or ultra violet radiation. Luminescence is observed with all phases of matter gases, liquids and solids both organic and inorganic1. Luminescence is an umbrella term for those kinds of events where light rise is not the result of mellowed temperature. In fact with increasing temperature the competing nonradiative de ardour processes become more probable and the intensity of the luminescent emission decreases. Luminescence in living organisms has been reported as far back as 1500 BC. In Chinese literature, the best known examples being the emission of light from fireflies and glow worms2, 3.1.1.1Types of LuminescenceClassification according to the federal a gency by which sinew is supplied to excite the luminescent molecule4-8.Table 1.1Table (1-1)- Different types of luminescence phenomenaExcitation sourceLuminescence typeProduced from irradiation tightness of Uv-vis or beneficial IR radiation.(Conversion of light energy to chemical energy).PhotoluminescenceEmission produced from irradiation of particles.CathodoluminescenceEmission produced from irradiation of particle.AnodoluminescenceRadiation-induced chemical excitation (irradiation of particles.RadiodoluminescenceProduced from heating.Emission from in corporationdescent solidsCandoluminescenceCaused by solid subjected to nuts heating.ThermoluminescenceFlame- crazy admixture atomsPyroluminescenceProduced from structural rearrangements in solidsEmission from shaking, rubbing, or crushing crystals.TriboluminescenceEmission from crystallization.CrystalloluminescenceExcitation induced from dissolution of an irradiated or other energy-donating solid.LyoluminescenceProduced from el ectrical phenomena.Radiative recombination of electrons and holes in visible (emission from electrical discharge)ElectroluminescenceEmission during electrolysisGalvanoluminescenceExcitation of compounds by ultra-sonication.SonoluminescenceEmission from fractional charge separation at the crystal surface.PiezoluminescenceProduced from chemical defendionsEmission from luminous organisms.BioluminescenceChemical excitation of compounds.Chemiluminescence1.2. ChemiluminescenceThe term chemiluminescence (CL.) was first coined by Eilhardt Weidman in 1888. Chemiluminescence is defined as the production of electromagnetic radiation observed when a chemical response yields an electronically worked up intermediate or product. CL-reactions emit light of varying degrees of intensity and life cartridge clip. With colors that spare the visible spectrum 9-11.Analytically, the CL-reactions are attractive due(p) to excellent detective work limits results from the absence of source of ruffle and scattering, suitable to both batch and flow techniques highly sensitive, determinable over a all-inclusive range of concentrations, easy to operate, simplicity, absence of unsuitable background so the only background subscribe is that of the PMT dark current and no excitation source or optical filters are required as compared to other optical or photo luminescence methods 12, 13. However, some disadvantages are to be considered as wellA chemiluminescence reagent may yield signifi mountaint emission not just for 1 unique analyte i.e. a lack of selectivity may occur.Moreover, chemiluminescence emission intensities are sensitive to a variety of environmental factors overmuch(prenominal) as temperature, solvent, ionic strength, pH and other species consecrate in the system. As a result, a separation condition (HPLC, capillary electrophoresis (CE), may not ever so match the optimum CL-emission conditions.Third, as the emission intensity from a CL-reaction varies with time,light flash composed of signal increase after reagent mixing, passing through a maximum, therefore decreasing back to the base line, thus the CL- emission versus time profile differs from one compound to another and care has to be taken to detect the signal in the flowing stream during strictly defined periods 14.chemiluminescence technique is versatile for determination of a wide variety of species that can participate in the CL-reaction such as CL-substrates or CL-precursors responsible for the aflame secernate 15, the necessary reagent for the CL-reactions (usually an oxidant) 16 , some species that enhance the rate or sensitivity of the CL-reaction17 ,activators such as catalysts (enzymes18 or metal ions19CL. analysis found a wide application not only in the organic technology, medicine, food testing, but also in environmental monitor and material science 20-24. CL-reactions can be coupled as a detection technique 25 in chromatography 26, 27, capillary electrophoresis 28-30, or immunoassay, providing qualitative and / or quantitative information 31, 32.1.2.1. Mechanisms of Chemiluminescence ReactionFigure (1-2) shows the types of the CL-reaction mechanisms. In general CL-reactions can be generated by two basis mechanismsFigure (1-2)- Types of CL-reactions. P, product F, fluorescing substance transport CL-reactions a substrate (A) and an oxidant (B) in the presence of cofactors react to form an intermediate product in an electronically huffy state which relaxes to the ground state with a photon emission. The catalyst is an enzyme or metal ions, reduces the activation energy and provides an adequate environment for producing high CL-efficiency out of the process.Cofactors sometimes are necessary to convert one or more of the substrates into a form capable of reacting and interacting with the catalyst,or to provide an efficient leaving group if bond cleavage is required to produce the excited emitter 33.An option for compounds producing ultra-weak CL-emissi on isIndirect or sensitized CL reactions Since not all excited states of CL-reactions products are effective emitters in any case, thus the excited state energy can be transferred to an effective flourophore added to the system, in order to increase the CL-intensity substantially . The most salient asset of chemical excitation is the low background noise involved, which results in very low detection limits (in the sub femtomole of region) 34,35.Enhanced CL- reactions (ECL) describes the phenomenon which has been observed in a wide range of chemi and bioluminescence reactions. Enhancement of light emission from the luminol- total heat bleach reaction at alkalescent pH has been achieved by the addition of various organic compounds generally containing oxonium group. Enhancement of CL. Reaction led to terrible light emission, draw out light emission and low background36. Spontaneous light emission during reaction requires the presence of the following conditions37The reaction es sential be highly exothermic (40-70 kcal/mole) in order that the CL-reaction occurred in the visible region (400 nm (violet) 750nm (red). These exothermic conditions associated with redox reactions using Oxygen, Ozone andhydrogen peroxide or similar potential oxidants.The reaction pathway must be a favorable to channel the energy for the formation of an electronically excited state.Photon emission must be favorable deactivation process of the excited product in relation to other competitive non-radiative processes that may appear in low proportion. There are several ways for an electronically excited state species to lose its extra energy, each by reacting with another molecule, molecular(a) dissociation, and collision with its surrounding molecules or luminescence. In the latter case it can react to liberate sufficient energy to produce photon of visible light, figure (1-3).Figure (1-3)-possible de-excitation pathways of excited molecule1.2.2. Main Chemiluminescence-systems for a nalytical processes1.2.2.1. Gas- phase chemiluminescence reactions.The development of Chemiluminescence Methods for determining components of a gas is largely originated from the need to determine atmospheric pollutants such as Ozone, Oxides of nitrogen and sulfur compounds45, 46.The best known and most frequently used is the reaction of nitrogen oxide withOzone.CL-NOXanalyzers have been shown to respond to a number of nitrogenous pollutants besides NO and NO2. 40. some(prenominal) analytical methods based on CL- reactions have been proposed for the determination of sulfur monoxide and unsaturated hydrocarbons such as Alkenes, alkynes and aromatics at high temperature,several of these are as follows41-44.NO+ O3NO2* + O2NO2 + h(600-2800 nm)(1)(2)(3)Oxygen atoms also been applied as strong oxidants when reacted with a wide range of analyst, although not so frequently used. The reaction with nitric oxide produces a yellow- green CL-emission that has been used for measuring concentratio n of Oxygen atoms in kinetics experiments45.(4)Some CL. Reactions are based on the light emission produced in flame, rather than the cold CL-emission. Molecular emission cavity analysis (MECA) is a CL-technique in which molecular emission are generated within a cavity that is introduced into a hydrogen diffusion flame. This flame acts as the environments of radicals, atoms and molecules which press the chemical reactions (i.e. high temperature of flame promote of chemical reactions that form key reaction intermediates and provide additional thermal excitation of the emitting species).The cavity acts as the environment for stabilizing the emitting species. MECA has been used for selective detection of compounds containing sulfur, nitrogen, phosphorous, antimony, arsenic,.Tin and halogens 46-54.1.2.2.2. Liquid- phase chemiluminescence reactionsThere are some well established CL-reagents that have been used for solution phase CL-reactions, their attraction include high sensitivity, wide slashing range and simplicity 55.The Chemiluminescence of luminal and the cyclic hydrazides of aromatic and heterocyclic acids is one of the classical and still studied in CL-reactions 56. Although hydrazides in general are Chemiluminescent, onlyhydrazides associate to luminol (5 aminophthalyhydrazide) can be considered the most commonly used. The CL. of luminol was first described by Albrecht in 1928. This compound reacts with potent oxidizing agents in the presence of catalyst in alkaline middling to yield 3 amino phthalate in an excited electronic state which returned to the ground state with the production ofblue emission at 425 nm.The reaction is catalyzed by metal ions.Alternatively, it has been proposed that the metal ion forms a complex with the hydrazide which is then oxidized5759.Lophin(2,4,5 triphenylimidazole) is the most representative of the imidazole CL-precursors . A yellow CL-emission is produced at 525 nm. by oxidisation of Luphin in sedimentary alkalin e mediums60.Lucigenine(10, 10 dimethyl 9, 9 -biascridiniumnitrate) is one of the most efficient CL-substances which emit an intense green emission at 470 nm. When oxidized in an alkaline medium 61.In relation to indirect CL., one of the more efficient non biological system that are used is based on and so called peroxy oxalate CL-reaction (PO CL ) which involves the hydrogen peroxide oxidation of an aryl oxalate ester in the presence of a fluorophore .Bis( 2,4,6 trichloro phenyl )oxalate (TCPO) and bis ( 2,4 dinitro phenyl ) oxalate (DNPO ) are commonly used oxalates. These compounds are insoluble in water which requires the use of organic solvents such as acetonitrilies ,dioxane and ethyl acetate 62.Another CL-system frequently applied involves the use of Ru bpy3+2 which produces an orange emission at 610 nm.From the excited state (Ru bpy3+2)* that can be obtained by different reactions which imply electron transfer and regeneration of Rubpy 3+2 species. Recently , the highe r CL-emission generated by a similar complex , Ruphen3+2(phen = 1,10 phenanthroline) during oxidation ofRu bpy 3+2 by Ce(IV) in sulfuric acid medium Ru bpy3+2 is the most studied as an inorganic compound used in electro generated CL (ECL)60,63.1.2.2.3.Solid Phase Chemiluminescence 35.Some polymers exhibit weak CL. in the visible range when heated up in the presence of inert gas (air or oxygen). For the first time this phenomenon was observed by Ashby in 1961, from polypropylene. Emission of light during oxidation of polymers is called oxy- luminescence or oxychemiluminescence. And for CL. of polymers to be observed, oxygen must be present in the process.1.2.3.Electro Generated Chemiluminescence (ECL)ECL is the process in accordance with which , species generated at electrodes undergo high energy electron transfer reactions to form excited states that emit light , it occurs in the visible region of spectrum as a consequence of fast and highly exo-energetic electron . Transfer bet ween a strong electron donor and electronacceptor which in turn results in generation of excited states 7.In general, electrically generated reactants diffuse from one or more electrodes, and undergo high energy electron transfer reactions either with one other or with chemicals in solution. This process yields excited state molecules, which produce a CL- emission in the vicinity of the electrode surface. Application of ECL have virtually exclusively focused on the reaction of tris(2, 2 bipyridire) ruthenium Rubpy3+2 the reaction takes place at room temperature . Wide pH range and in the presence of oxygen and many other impurities64.Ru bpy3+2 itself can undergo electron transfer reactions to produce ECL as follows 65.Luminol can undergo an ECL reaction due to its versatility in analytical determinations. in alkaline solution luminol anion undergoes a single electron electro-oxidation to form diazaquinine , which is further oxidized by peroxide or highly oxide to give 3 amino p hthalate in an excited state , which emits light at 425 nm . Luminol ECL has been used for determination of species such as phenyl alanine, ibuprofen and hisidine 9.1.3. FluorescencePhotoluminescence becomes visible as optical radiation accompanied by the emission of photon depending on the duration of the phenomenon, i.e. life time of the excited state.Fluorescence is the spontaneous emission of light during transit of the system from its lowest vibrational energy level of an excited singlet state S1 back to the ground state S0,thus its a spin allowed process. Absorption and fluorescence dont require any spin reorientation however intersystem crossing and phosphorescence require a spin reorientation therefore absorbance and fluorescence are much faster than phosphorescence. The processes that occur between the absorption and emission of light are usually illustrated by Jobalonski diagram 66, 67 figure (1-4).Figure (1-4)- Perrin- Jobalonski diagram and illustration of the relation back positions of absorption (Abs), fluorescence (F), andphosphorescence (Ph.)Fluorescence is the result of three stages process that occurs in certain molecules called fluorophores 67.Excitation a photon is supplied by an external source and absorbed by the fluorophore creating an excited electronic singlet state S1. This stage distinguishes fluorescence from chemiluminescence, in which the excited state is populated by a chemical reaction.State life timefluorescence life time is typically 1-10 Nano sec. during this time the fluorophore undergoes conformational changes and is also subjected to a multitude of possible interactions with its molecular environment such as collisional quenching and intersystem crossing.Fluorescence emission when a photon of energy is emitted reversive the fluorophore to the ground stat S0. Figure (1-5) shows these three stages.Figure (1-5)- three stages of fluorescence emission process1.3.1. Characteristics of fluorescence emission 68-70Emission spect ra are typically independent of the excitation wave aloofness due to the partial dissipation of excitation energy during the excited state life time, this known as Kashas rule. The emission intensity is proportional to the bounty of the fluorescence excitation spectrum at the excitation wavelength figure (1-6).Figure (1-6)- emission spectra are typically independent of the excitation wavelengthStokes shift due to the energy dissipation during the excited state life time, the energy of this photon is lower and therefore of longer wavelength than the excitation photon (i.e.fluorescence light is a red-shift). This difference in energy or wavelength is known as stokes shift. The difference in wavelength between positions of the band maxima of the absorption and emission spectra of the same electronic transition figure (1-7). Solvent effects and excited state reactions can also affect the magnitude of stokes shift.Figure (1-7)-stokes shift of band maximaE = h = hc / (1)h = Planck cons tant = 6.63 10-34 J.sec-1 c = light upper = light frequency Sec-1 or Hz = wave length nm.Absorption energy EAbs.= hc / Abs. (2)Emission energy Eem.= hc / em. (3)Since EemAbs. (4)so em.Abs (5)em.,Abs emission and absorption spectra peaks respectively.When absorption and/or the emission spectra of a flourophore possess two or more bands, stokes shift is equal to the difference that separates the two most intense bands of the two spectra, figure (1-8). Figure (1-8)-stokes shift in a flourophore possess two bandsIf thermal energy is added to an excited state or compound has many highly populated vibrational energy levels, emission at shorter wavelengths than those of absorption occurs, this is called an anti-stokes shift and its often observed in dilute gasses at high temperatures.A key feature of fluorescence is the molecule spends a measurable Amount of time in the singlet state. Fluorescence life time refers to the average time that the molecule waistband in its excited state b efore emitting a photon for fluorescence its typically in the range (1-10 nsec.).Fluorescence radiant power (F) is proportional to the absorbed radiant power.F = (P0 P) - (6) Fluorescence efficiency. P0 incident power.P transmitted power.The relationship between the absorbed radiant power and concentration can be obtained from beers lawP/P0 = 10-A- (7) P = P0 10-bc - (8)F = P0 (110-bc)- (9)Structural factors. Fluorescence is expected in molecules that are aromatic or multiple conjugated double bonds with a high degree of resonance stability, its also expected with poly cyclic aromatic