The mechanism of the photoluminescence changes in bio-conjugated CdSe/ZnS quantum dots
Highlights
► The changes in luminescence and Raman spectra in CdSe/ZnS quantum dots (QDs) during storage have been studied. ► The QD luminescence band blue shifts, broadens and decreases in intensity. ► The CdSe (LO) peak shifts to the low frequency region and broadens. ► The changes are ascribed to QD oxidation, which decreases the QD size. ► Bio-conjugation is supposed to promote QD oxidation.
Introduction
Today, one of the most relevant applications of colloidal II–VI quantum dots (QDs) is in fluorescence bio-sensing and bio-imaging [1], [2]. The unique optical and chemical features of the QDs allowed them to overcome some of the functional limitations encountered with organic dyes. This opened wide prospects for applications of the QDs in biology and medicine where long-term, multi-target and highly sensitive imaging is required. It is anticipated that QDs could be successfully used for ultrasensitive and multiple-target protein detection instead of organic dye fluorophores in different immunofluorescence assays including Enzyme-Linked ImmunoSorbent Assay (ELISA) [2]. Specifically, it has been shown that sensitivity of a “sandwich” ELISA cancer testing technique can be improved by application of the effect of a spectral shift of the photoluminescence (PL) band in CdSe/ZnS core–shell QDs conjugated with target cancer antigens [3]. However, the mechanism of this effect is still remaining not completely elucidated.
The effect of a spectral shift has been found to arise upon drying of the QDs dispersed in a buffer solution on a crystalline Si wafer [4]. The shift magnitude increases during storage of the dried QDs at atmospheric ambience for several days. In bio-conjugated QDs, it increases much faster than in non-conjugated ones. This enables to distinguish between PL originating from bio-conjugated QDs and that from non-conjugated ones. This can be used for improvement of the sensitivity of the QD luminescent tagging techniques.
The effect of a spectral shift was observed in the CdSe/ZnS and CdSeTe/ZnS core–shell QDs conjugated to different monoclonal antibodies [5], [6]. It was supposed to be caused by an increase of compressive elastic strains in the QDs upon drying. Besides, in the samples stored in the atmospheric ambience for long time (up to 2 years), an oxidation of QD core has been found to be responsible for significant difference in the PL peak position between the bio-conjugated QDs and non-conjugated ones [6]. It has been shown that the process exhibited as the PL spectral shift can be facilitated noticeably and the shift magnitude can be increased by annealing of the samples at elevated temperatures for hours [4].
Here, we show that visible light illumination of dried QDs during their storage affects their properties similarly to the annealing. This can be used as an effective processing to change the color of the emission from bio-conjugated QDs faster in comparison with that from the non-conjugated ones. The evolution of the PL and Raman scattering spectra of dried non-conjugated and bio-conjugated CdSe/ZnS core–shell QDs kept in the darkness and under illumination in the atmospheric ambience is presented.
Section snippets
Experimental details
Commercial water-soluble CdSe/ZnS core–shell QDs covered with an amphiphilic polymer and polyethylene glycol (PEG) purchased from Invitrogen Inc. [7] were used. The QDs were ellipsoids with minimum and maximum axis of 6 and 12 nm, respectively [7]. The QDs were conjugated to mouse monoclonal antibodies against ribosomal protein S6 kinase 2 (S6K2). S6K2 is known to participate in various cellular processes, including mRNA processing and protein synthesis, as well as in some pathologies, e.g.,
Experimental results
The evolution of conventional PL spectrum in the C-QDs when they are kept under day light illumination is shown in Fig. 1a. During the storage the PL band shifts to the high energy region from 1.9 to 1.95 eV (blue shift), its full width at a half maximum (FWHM) increases from 86 to 130 meV and the PL intensity becomes two times smaller. These qualitative changes are observed in the PL spectra of both NC-QDs and C-QDs, but for the C-QDs they are larger. Besides, the transformations of the PL
Discussion
The investigation of the PL and Raman scattering spectra of stored QD-samples shows that the storage of dried QDs promotes transformation of both PL and optical characteristics. The PL changes consist in a blue shifting of PL peak position, increasing FWHM and decreasing intensity of the PL band. The transformations in the optical characteristics consist in the shifting of the CdSe (LO) peak position to the low frequency region and the increasing of its FWHM. The fact that the same correlations
Conclusions
The PL and Raman spectra in non-conjugated and conjugated with S6K2 antibodies CdSe/ZnS core–shell QDs dried on a Si substrate and stored in the darkness and under visible light illumination for up to four months have been investigated. It is found that storage results in a blue shifting of the PL peak position, in the increasing FWHM of the PL band and in decreasing of the PL intensity. These transformations are accompanied by shifting of the CdSe (LO) peak position to the low frequency region
Acknowledgements
This work was partially supported by the National Academy of Sciences of Ukraine through the program “Nanotechnologies and Nanomaterials” (grant no. 2.2.1.14/26) and the project “Physical and Physical-Technological Aspects of Fabrication and Characterization of Semiconductor Materials and Functional Structures for Modern Electronics” (grant no. III-41-12).
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