A new nanocomposite fluorescence probe with thioglycolic acid (TA) functional layers embedded inside the hydroxyapatite nanoribbon spherulites has been synthesized. The fluorescence intensity of the novel probe is about 1.5–3.3-fold increase compared with the probe containing no TA. When used to detect cadmium ion, the most of original assembly nanoribbon spherulites structure in the novel probe is found to have been damaged to new flake structures. The mechanism of determining cadmium ion in alcohol solution has been studied. The present systematic study provides significant information on the effect of assembly nanostructure on the metal-enhanced fluorescence phenomenon.
KeywordsComposite materials Nanocomposite fluorescence probe Cadmium ion Hydroxyapatite
Hydroxyapatite (Ca10(PO4)6(OH)2, HAP) is a calcium phosphate salt and being used as an artificial bone material due to its biocompatibility, bioactivity, osteoconductivity, nontoxicity, nonimmunogenicity, and noninflammatory behavior. . Current preparation methods of HAP chiefly include conventional wet chemical , template synthesis , calcination of xenogenic bone , and hydrothermal conversion of calcium carbonate exoskeleton . The properties of HAP differ with preparation route, reaction ingredients, etc. Among many properties, crystallographic changes directly affect the activity of HAP; thus, it is necessary to study its morphology and crystallography before practical application [6, 7].
Many studies have recognized the ability of HAP to bind divalent heavy metal ions and have shown that synthetic HAP has a high removal capacity for Pb, Zn, Cu, Cd, Co, and Sb in aqueous solutions [8–20]. Mechanisms such as dissolution of HAP and precipitation of metal phosphates [10, 11], surface complexation [11, 12], ion exchange , substitution of Ca in HAP by metals during coprecipitation  have been proposed in order to describe the uptake of heavy metals from aqueous solutions by synthetic HAP. It is difficult to detect the formation of Cd complexes or modification of the structure of HAP by Cd due to the similar ionic radii of Ca (0.99 Å) and Cd (0.97 Å). Moreover, few studies about microanalysis in organic compounds by nanotechnology have been reported. Therefore, the corresponding research will be explored in present work.
The HAP nanoribbon spherulites, which had been obtained in our previous studies, have large surface, good bio-consistency and high physical/chemical activities . Because of a lot of hydroxy groups in their surface, the HAP nanoribbon spherulites can be modified to prepare probe to detect heavy metals in organic compounds [22, 23]. In this paper, we have prepared a novel HAP nanocomposite for developing a simple and rapid approach to increase the fluorescence sensitivity of Cd2+ in alcohol solutions.
The Preparation of Hydroxyapatite Nanoribbon Spherulites
The HAP nanoribbon spherulites were prepared according to our previous study showed in reference . The eggshell membrane was made by removing the outer shell of a fresh eggshell and washing with deionized water. Then it was fixed in a container to separate it into two horizontal compartments, into which 20 mL of 0.1 mol/L CaCl2 and 20 mL of 0.06 mol/L KH2PO4 solutions were added, respectively. The ethylenediamine was put on both sides of the membrane to modulate the pH value to 7.4. All reagents used were analytical purity class. Thereafter, the reaction container was kept at room temperature for about 10 h. The product was obtained by centrifugal separation, then washed with deionized water and dried in a desiccator.
Typical Preparation of Nanocomposite Fluorescence Probe
About 0.005 g of the above HAP product and 5.0 μL thioglycolic acid (TA) were put in a beaker with 5.0 mL alcohol solution. Then the system was vibrated ultrasonically for 2 min. After that, the reaction system was centrifugated at 1,500 rpm for 10 min, and the nanocomposite fluorescence probe settled to the bottom of the container.
The morphologies of the products were investigated through transmission electron microscopy (TEM, Hitachi-800) and scanning electron microscopy (SEM, Philips S-4800). The optical properties of the nanocomposite fluorescence probe in alcohol solution were studied through fluorescent spectroscopy (Perkin-Elmer LS-55) and Fourier transform infrared spectroscopy (FT-IR).
Results and Discussions
Morphologies of Products
And the assembling bondings of nanoribbons were broken by the transmitting energy in the process of reaction, the probe with spherulites shape transformed to separation flakes. The more exact mechanism has not claimed and it will be conducted in our further exploitation.
Fluorescence Phenomenon of Probe in Alcohol Solution
Enhanced Fluorescence Intensity of Probe by Cadmium Ion
Appropriate Concentrations of TA and HAP
Because the interactions among TA, HAP, and cadmium ions are complicated, we should select the appropriate concentrations of TA and HAP in order to get the appropriate fluorescence intensity. In all solutions, the solvent is alcohol.
FT-IR Analysis of Probes
This work was supported by the State Key Laboratory of Pollution Control and Resource Reuse Foundation, China (NO.PCRRF09005).
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