Fluorescence And Photo Electrochemical Sensors Based On Nanomateirals

Quantum dots have unique and superior optical and photoelectrochemical property,which make them a new class of materials. In recent years，quantum dots have gainedincreasing attention and played important roles in the fields of developing fluorescent andphotoelectrochemical sensors.The fluorescence and photocurrent of quantum dots are extremely sensitive to theirsurface states. The direct interaction between certain analytes and the surface of quantum dotscan influence the efficiency of the electron-hole recombination process. On this basis, simpleand sensitive fluorescence and photoelectrochemical sensors were developed.However, most of fluorescence sensors were found based on quenching the fluoresenseof quantum dots. In comparison with the fluorescence quenching mode, where a variety offactors rather than analytes can induce the ultimate fluorescence“off”state, the fluorescence“turn-on”mode seems to be more preferable due to the reduction of the chance of falsepositives. Two“turn-on”mode fluorescence sensors were developed in this report.Photoelectrochemistry, which has the advantage of simple, cheap and high sensitivity,was a newly developed analytical method. As we know, the quantum dots show excellentphotoelectric chemical activity due to the surface effect. In this paper, thephotoelectrochemical sensor was developed based on quantum dots for determination ofdopamine.The main research contents of this article are as follows:1. A sensitive and simple method for the determination of melamine （MA） wasdeveloped based on the fluorescence enhancement effect of MA for thioglycolic acid（TGA）-capped CdS quantum dots. Under optimum conditions, a good linear relationship wasobtained from2.0×109to5.0×105mol/L. The detection limit is1.0×109mol/L, which ismuch lower than the safety limit （2.5ppm in USA and the UK;1ppm for infant formula inChina）. The solution pH, the adding sequence of the buffer solution and MA and surfacemodifiers of CdS quantum dots greatly influenced the enhancement extent of MA for CdSquantum dots. The fluorescence enhancement was attributed to the surface passivation of thesurface states of quantum dots by amine group of MA. The method was applied to detect MAin raw milk with satisfactory results. The proposed method manifested several advantagessuch as high sensitivity, short analysis time, low cost and ease of operation.2. A novel switchable sensor was developed for the determination of phosphate based onCe³⁺induced aggregation and phosphate triggered disaggregation of cysteine （Cys）-cappedCdS quantum dots and silver nanoparticles （AgNPs）. Rare earth of Ce³⁺could aggregate themixture of quantum dots and AgNPs, which induced electron or energy transfer between CdSquantum dots and AgNPs and serious fluorescence quenching. However, phosphatedissociated the formed aggregation of CdS quantum dots and AgNPs, restoring the enhancedfluorescence of Cys-capped CdS triggered by AgNPs. Although, CdS quantum dots alonecould also be used to detect phosphate through the aggregation-disaggregation mechanismadjusted by Ce³⁺and phosphate. It was found that the distance-dependent interaction between AgNPs and CdS quantum dots driven by Ce³⁺and phosphate could lead to enhancedquenching or enhancement of the fluorescence of Cys-capped CdS to form more sensitivedetection system for phosphate. The developed method was applied to detect phosphate inreal water samples with acceptable and satisfactory results.3. A highly sensitive photoelectrochemical sensor was developed based on CdS quantumdots for the determination of dopamine. Dopamine could reduce the photocurrent of CdS afterdopamine was oxidized. In alkaline condition, dopamine could be oxidized by O2. Theoxidation product autopolymized to form polydopamine. In neutral condition, dopamine couldbe oxidized by electrochemical method. The oxidized dopamine could form polydopaminefilm on the surface of electrode. The polydopamine film could hinder the electron transferfrom CdS to the surface of electrode and the oxidation product of dopamine could shuttle theelectron to valence band of excited CdS, inducing the photocurrent intensity of CdS decrease.However, in alkaline condation, only part of polydopamine would be adhered to the surface ofelectrode. Therefore, the degree of photocurrent decrease of CdS when using electrochemicalmethod to oxidize dopamine was higher than that when using chemical mehod to oxidizedopamine in neutral solution. The coupling of electrochemical and photoelectrochemicalmethod showed higher sensitivity for determination of dopamine.