چكيده به لاتين
The present thesis is organized through five research project. In the first project, a fluorescence sensor based on carbon quantum dots (CDs) was used for caffeine determination. The fluorescence of CDs decreased in the presence of copper ion (Cu2+) due to dynamic quenching. Upon addition of caffeine, the fluorescence of CDs was recovered due to the strong affinity between Cu2+ and caffeine. The recovery of the fluorescence intensity was used as means for the quantitative evaluation of caffeine concentration. The effective parameters in sensing process, including time, temperature, pH and concentration of CDs, were optimized using central composite design. Under optimal conditions, the linear response range of this sensor for caffeine was from 0.2 μM to 70 μM. The limit of detection of caffeine was 0.05 μM. There were several advantages of this fluorescence sensor including high sensitivity, lable free, easy operation and acceptable selectivity.Finally, the carbon quantum dots -based fluorescent probe was successfully employed for the caffeine detection in food sample.
In second project, a nitrogen and sulfur co-doped graphene quantum dot (N,S/GQDs) for the detection of ethion were synthesized by a one-pot hydrothermal method.By optimizing synthesis conditions, the N,S/GQDs with a high PL QY of 22.3% were achieved. The fluorescence of N,S/GQDs is quenched as a result of electrostatic interactions with the Hg2+ ions absorbed on the negatively charged N,S/GQD surface. In the presence of ethion, the fluorescence of the quenched N,S/GQDs is recovered. The mechanism was studied using the Stern–Volmer equation and zeta potential measurements. The linear range for the detection ethion was attained over the concentration range from 19.2 to 961.2 μg/L with a detection limit of 8 Μg/L. The interfering effects of some foreign ions and practical application of this method in real sample was investigated. In third project, using silver sulfide quantum dots (Ag2S QDs) and graphitic carbon nitride nanosheets (g-C3N4), an effective and facile ratiometric fluorescence sensor for the determination of Cerium was designed. The synthesized products were characterized using several methods such as FT-IR, TEM, XRD, PL and EDS. The addition of cerium ions to the probe solution quench the fluorescence of g-C3N4 at 443 nm, while they enhance the fluorescence of Ag2S QDs at 540 nm via aggregate formation of Ag2S QDs. Using TEM imeage and measuring the hydrodynamic size, the proposed mechanism was confirmed. The analytical potential of the probe was evaluated and obtained a good linear range for Cerium concentration of 0.5 μM to 32 μM with a detection limit of 64 nM.
In fourth project, Arginine–functionalized graphene quantum dots (Arg-GQDs) were successfully synthesised from citric acid in the presence of Arginine. The Arg-GQDs exhibited a maximum emission at 448 nm with an excitation wavelength of 350 nm. The obtained Arg-GQDs were used as a novel “off-on” fluorescent probe for the determination of thiamine with high sensitivity and selectivity. The effect of different parameters were optimized by central composite design (CCD) combined with response surface methodology (RSM). Under optimal conditions, the fluorescence senor showed a sensitive response to thiamine in the concentration range of 0.1–8.0 M with a detection limit of 53 nM. The proposed optical sensor was applied in the detection of thiamine in food samples.In fifth project, a optical sensor based on immobilization of lignosulfonate−graphene oxide- polypyrrole on triacetylcellulose film was designed for the determination of Pb(II) ions in aqueous solution. A dynamic working range, detection limit, sensitivity, selectivity and the response time were discussed in detail. The response time of the sensor was within 100 s depending on the concentration of Pb(II) ions. The sensor was applied to determine the Pb(II) ions in water samples.
