Fluorescence enhancement of CdTe MPA-capped quantum dots by glutathione for hydrogen peroxide determination S. Sofia M. Rodrigues a , David S.M. Ribeiro a,n , L. Molina-Garcia b , A. Ruiz Medina b , João A.V. Prior a , João L.M. Santos a a Requimte, Department of Chemical Sciences, Laboratory of Applied Chemistry, Faculty of Pharmacy, University of Porto, Rua de Jorge Viterbo Ferreira no 228, 4050-313 Porto, Portugal b Department of Physical and Analytical Chemistry, Faculty of Experimental Sciences, University of Jaén, Campus las Lagunillas, E-23071 Jaén, Spain article info Article history: Received 25 November 2013 Received in revised form 21 January 2014 Accepted 23 January 2014 Available online 31 January 2014 Keywords: Fluorescence enhancing Surface passivation Glutathione CdTe quantum dots Flow analysis. abstract The manipulation of the surface chemistry of semiconductor nanocrystals has been exploited to implement distinct sensing strategies in many analytical applications. In this work, reduced glutathione (GSH) was added at reaction time, as an electron-donor ligand, to markedly increase the quantum yield and the emission efficiency of MPA-capped CdTe quantum dots. The developed approach was employed in the implementation of an automated flow methodology for hydrogen peroxide determination, as this can oxidize GSH preventing its surface passivating effect and producing a manifest fluorescence quenching. After optimization, linear working calibration curve for hydrogen peroxide concentrations between 0.0025% and 0.040% were obtained (n¼6), with a correlation coefficient of 0.9975. The detection limit was approximately 0.0012%. The developed approach was employed in the determination of H 2 O 2 in contact lens preservation solutions and the obtained results complied with those furnished by the reference method, with relative deviations comprised between 1.18 and 4.81%. & 2014 Elsevier B.V. All rights reserved. 1. Introduction The surface chemistry of colloidal semiconductor nanocrystals or quantum dots (QDs) is an important parameter determining most of their optical and physical properties, namely their reactivity, lumi- nescence efficiency (quantum yield), photoluminescent properties stability and the solubility in a given solvent [1]. Indeed, the QDs size (usually in the range 1–10 nm) and the resultant quantum confine- ment effect in combination with the high surface-to-volume ratio and other surface characteristics render QDs photoluminescent properties very sensitive to any micro-environmental change or interaction with a chemical specie [2]. Morphologically, the occur- rence of surface imperfections that act as charge carrier traps can impair the efficiency of electron–hole recombination, thus favoring non-radiative recombination processes which can dramatically reduce the fluorescence quantum yield (QY) of QDs [3]. In this regard, surface modification strategies are often used to eliminate trap sites and to increase solution stability preventing aggregation and leading to an enhancement of the QDs luminescent emission intensity [4]. Organic ligands used in the adaption of the surface chemistry provide electronic and chemical passivation of surface traps and enable QDs to be chemically manipulated as large molecules with solubility and reactivity defined by the ligand characteristics [5]. This adaptability has boosted the application of QDs as chemosensors in different analytical applications gaining a wide acceptance among the scientific community. Several works have studied the interaction of quantum dots surface with different substances exhibiting functional groups such as thiol (sulfhydryl) [3,6], amine [7] and phosphonate [4] that could act as enhancers of the fluorescence intensity of QDs. This enhancement was explained by the formation of covalent bonds between the donor atom of the ligand (usually nitrogen, sulfur or oxygen) and incompletely coordi- nated Cd 2 þ ions on the QDs surface, wherein the referred atoms acted as electron-donors and the dangling orbitals acted as electron- acceptors. Thus, the mid-gap energy states produced by dangling orbitals located between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are effectively removed, thereby preventing the occurrence of non- radiative relaxation pathways. Glutathione (L-γ-glutamyl-L-cysteinylglycine) is the most abun- dant intracellular non-protein sulfhydryl compound present in all mammalian tissues participating in numerous cellular functions mainly involving the thiol group of the cysteine residue [8]. In particular, reduced glutathione (GSH) plays an important role in detoxification of hydrogen peroxide, other peroxides, and free radicals [9]. Furthermore, GSH has been widely used as a thiol ligand in the aqueous synthesis of different semiconductor nano- crystals [10,11]. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/talanta Talanta 0039-9140/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.talanta.2014.01.031 n Corresponding author. Tel.: þ351 220428664. E-mail address: dsmribeiro@gmail.com (D.S.M. Ribeiro). Talanta 122 (2014) 157–165