The Phase Transformation Of Calcium Phophates And Its Applications In Electrochemical Analysis

Calcium phosphates?Ca Ps?are the main inorganic constituents of bone and tooth.A great variety of Ca Ps phases including hydroxyapatite(HAP,Ca₁₀?PO₄?₆?OH?₂),calcium hydrogen phosphate?DCPD,CaHPO₄·2H₂O?,OCP,Ca₈?HPO₄?₂?PO₄?₄·5H₂O),amorphous calcium phosphate?ACP?have been studied extensively.Especially,Ca Ps could be used as promising biomaterials in the biomedical field inspired by its phase transition tuned by the intermediate coordinated water?H₂O?.The unexplored electrooxidation process of the coordinated H₂O attracts our interest.Meanwhile,one unexpected observation for the electrochemical response and electrochemiluminecence?ECL?during the calcium phosphate transition further triggered this exploration.In this dissertation,the phase transformation of Ca Ps and its electrochemical feature have been revealed,and the Ca Ps based electrooxidation and ECL platforms further have been developed for the determination of phosphate and organic molecules.The main achievements are listed as follows:?1?The phase evolution of Ca Ps in the presence of ionomer Nafion were presented,and the investigation will exhibit one fundamental for the Ca Ps electrochemical study.In the preliminary stage of Ca Ps formation with the initial p H 9,OCP was the major forms of Ca Ps in the presence of high calcium ions(Ca²⁺)source?Ca/P=0.2M/0.1M?.In contrast,DCPD would be the main product with low Ca²⁺ions source?Ca/P=0.1M/0.2M?.The ion exchange feature and morphology of Nafion could tune the Ca Ps formation and phase evolution.When the initial p H of the incubating dispersion was adjusted to 5,monetite?CaHPO₄,DCPA?was obtained in the presence of Nafion,the product defers from the DCPD formed from the solution in the absence of the ionomer.Under alkaline condition,Nafion play an essential role to facilitate deprotonation process to extend the transition for HAP formation via DCPD dissolution-reprecipitation process.And the rod-like HAP was formed by the induction of the elongated pattern Nafion with the ion exchange behavior of its side chain.?2?The electrochemical oxidation behavior during the Ca Ps phase transition was investigated,and the direct electrochemical sensing platform for phosphate ions was developed.The mixture of ACP and OCP,which acts as the starting material for HAP,was self-assembled on Nafion modified glassy carbon electrode?GCE?.The as-prepared electrode?Ca Ps/Nafion?showed a distinct oxidation peak at 1.0 V vs Ag/Ag Cl in phosphate solution.The peak heights were directly proportional to the concentration of phosphate from 0.1?M to 10?M in the presence of 1 m M Ca²⁺.After the comprehensive characterization of Ca Ps/Nafion electrode,it was understood that the phosphate ions were used as the proton acceptors which could stimulate the generation of coordinated hydroxy?-OH?from coordinated H₂O during the Ca Ps phase transformation.And the addition of Ca²⁺ions could further to promote the Ca Ps/Nafion electrochemical response for the phosphate ions due to its hydarion to H₂O.The Ca Ps/Nafion electrode also displayed good selectivity as the electrochemical oxidization response was not affected by up to 10?M of the potentially competitive species like CO₃^2-,NO₃^-,CH₃COO^-,SO₄^2-,Cl^-.?3?Singlet oxygen?¹O₂?electrochemiluminecence?ECL?was observed during the phase transition from DCPD to HAP in situ assembled on Nafion film,and the machnism was also proposed.The nanostructured DCPD was assembled in situ on Nafion modified glassy carbon electrode?GCE?in the solution containing saturated Ca²⁺and phosphate ions.Upon application of cyclic voltammetric?CV?scan for the DCPD assembled by Nafion,a strong electrogenerated chemiluminescence?ECL?was observed at the oxidation peak at 1.0 V vs Ag/Ag Cl in p H 7.4 phosphate buffer solution.Characteristic emission signals at 634 nm and1268 nm verified the generation of singlet oxygen.Raman spectroscopy and X-ray diffraction?XRD?spectrum evidenced phase transformation during CV scans,and entailed CV peaks showed the transformative immediate species were attributed to the ECL.This finding provides one insight into the intermediate species during the Ca Ps phase transformation.?4?The possible mechanism of Ca Ps ¹O₂ ECL tuned by organic molecules was investigated,and some organic molecules assay based on the ¹O₂ ECL was explored.ACP is considered as a classic precursor of HAP,was assembled on Nafion coated GCE?denoted as ACP/Nafion?.And its electrochemical oxidation peak and ECL phenomenon were observed.The characteristic spectrum at 634 nm verified that the ECL derived from the¹O₂ dimol emission.And the ECL intensity could be dramatically increased and stabilized in the presence of organic molecules,e.g.,isopropanol?IPA?,oxalate(C₂O₄^2-)and L-Lysine.The electron spin resonance?ESR?measurement confirmed that plenty of relevant radicals were produced during the organic molecules involved electrochemical oxidation,and these radicals could be believed to have played the essential role for the ¹O₂ generation.Furhermore,the assays for IPA and L-Lysine based on ACP/Nafion ¹O₂ ECL have also been developed.The results show that IPA and L-Lysine can be detected in the range 1.0?M to 1.0 m M and0.1?M to 0.1 m M,and the respective limit of detection?LOD?were 0.15?M and 0.03?M.This work inspired an insight to the organic molecule enhanced ¹O₂ ECL,and exhibits the potential for the development of widely applicable ECL detection platform based on abundantly available materials.