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V. Poderys et al. / Medical Physics in the Baltic States 7 (2009) 24 - 29 have photoluminescence band at 600 nm is 3.5 nm [10]. It is approximately equal to height of one layer. This confirms idea, that precipitate formed in solution is large aggregates formed of CdTe quantum dots. Measurements of quantum dots deposited from freshly prepared solutions (Fig. 4 A-C) also shows, that quantum dots forms aggregates. AFM image of quantum dots, deposited from solution that was kept for 40 min., is presented in Fig. 4 A. A lot of small round structures are present on the surface. These structures are 2 nm – 3 nm in height and 20 nm – 30 nm in width. Photoluminescence intensity, a.u. Absorbance, o.d.u. Photoluminescence intensity, a.u. 800 600 400 200 0 0.3 0.2 0.1 0.0 800 600 400 200 0 A PL intensity λ max Δλ FWHM 0 100 200 300 B Time, h 600 56 λ , nm max 580 560 540 0 100 200 Time, h 300 C PL intensity λ max Δλ FWHM 56 λ FWHM , nm 52 48 0 100 200 300 Time, h 52 48 44 40 36 580 570 560 550 Δλ FWHM , nm λ max , nm 27 Absorbance, o.d.u. 0.3 0.2 0.1 D 0.0 0 100 200 300 Time, h Fig. 3. CdTe QD spectral properties dependence on time in the presence of BSA. Photoluminescence band intensity, position and width dynamics: A – in deionized water with BSA, C – in saline with BSA. Quantum dot absorption band intensity dynamics: B – in deionized water with BSA, D – in saline with BSA. (Concentration of quantum dots c=3,84·10 -7 mol/l, concentration of BSA c=3·10 -5 mol/l). Height of these structures is approximately equal to height of single quantum dot. Shape of colloidal quantum dots should be close to spherical. Structures seen on the surface have much bigger width than height. This can be explained by AFM imaging artefact called “tip imaging”. It is also possible that that these small structures are not single quantum dots but few quantum dots that are attached to each other. AFM image of quantum dots deposited from solution that was kept for 5 hours shows larger structures (fig. 4 B). After 5 hours height and width of structures varies in broader range. Some small structures (height - 3 nm, width – 20 nm) can be seen but also bigger structures (up to 9 nm in height and up to 70 nm in width) appear. Image of sample prepared from solution that was kept for 24 hours (Fig. 4 C) shows that sizes of these structures increase even more. All these samples were prepared under the same conditions, so changes in the images are caused by changes in the quantum dot solution. Fig. 4 A– C shows that after dissolving quantum dots in deionized water, quantum dots start to aggregate and after some time precipitate of large aggregates appear. In case of quantum dots solutions with BSA, precipitate does not appear even after long time. Image of samples prepared from quantum dots in saline with BSA taken 1 month after solution preparation (Fig. 4 E) shows that there are no large structures that could form precipitate, but there are plenty of round structures that are 9 nm – 20 nm in height and 40nm – 60 nm in width. This result confirms that protein stabilizes quantum dots in aqueous solution and prevents quantum dots from aggregation.
E D A B C V. Poderys et al. / Medical Physics in the Baltic States 7 (2009) 24 - 29 Fig. 4 AFM images of CdTe quantum dots (A-D) and CdTe quantum dots with BSA on mica. CdTe quantum dots deposited from solution in deionized water: A – 40 minutes after preparation, B – 5 hours after preparation, C – 24 hours after preparation. D – CdTe quantum dots deposited from saline solution after precipitate was formed (insert – phase image). E – CdTe quantum dots with BSA deposited from saline solution 1 month after preparation of solution. (A–C and E images are 2 μm x 2 μm, image D – 5 μm x 5 μm). We also measured photoluminescence kinetics of quantum dots solution in deionized water with and without BSA. Photoluminescence decay of both samples is presented in Fig. 5. Photoluminescence of quantum dots without protein decays faster (Fig.5 line A) and four photoluminescence decay lifetimes are needed to get good decay fit (reduced χ 2 =1.018). These lifetimes are: τ1=3.4ns, τ2=14.1ns, τ3=30 ns, τ4=88,2 ns. Fitting with tree exponents doesn’t give good results – χ 2 =1.268. In case of quantum dots solutions with BSA, three exponents are enough to get good fitting results (χ 2 =1.063, life times - τ1=10.5ns, τ2=29.6ns, τ3=78.9ns). 28 A Fig. 5. Photoluminescence decay kinetics of CdTe quantum dots in deionized water (measured 48 h after preparation): line A – without BSA, line B – with BSA. 4. Discussion Dynamics of absorption and photoluminescence properties of investigated solutions (presented in Fig. 1 and Fig. 3) shows two phases – growth of photoluminescence and decrease of photoluminescence. In the first phase photoluminescence of quantum dots increased in all investigated solutions. Despite quite large increase in photoluminescence spectra, changes in absorption spectrum are very small an even a small decrease of absorption band can be seen after 24 hours. After that absorption of quantum dots solutions starts slowly increasing. During this phase photoluminescence band peak position and width remain constant. These changes indicate that core of quantum dot remains intact. Core degradation would cause blue shift of photoluminescence band; aggregation of quantum dots would cause a red shift. Change of photoluminescence intensity indicates, that properties of quantum dots coating (or coating itself) is changing: molecules coating core of quantum dot are rearranging, being replaced by other molecules of being washed-out. Theoretically increase of quantum dots photoluminescence intensity is explained by decrease of nonradiative transitions or their speeds. Decrease of defects on quantum dots surface would give this effect [11]. Another process that can change intensity of quantum dots photoluminescence is aggregation. Aggregation of quantum dots decreases photoluminescence quantum yield. Slow dissolution (monomerisation) of quantum dots powder (aggregates) could cause increasing photoluminescence intensity due to increased photoluminescence quantum yield of monomeric quantum dots compared with aggregated form. More detail investigation of absorption spectrum dynamics during first two days after preparation of solution (Fig. 1 B insert) contradicts to this explanation. Absorption of quantum dots dissolved in deionized water decreases during first day. This decrease can be explained by aggregation of quantum dots. Aggregation of quantum dots leads to decrease of absorption intensity, red shift, broadening and intensity decrease of photoluminescence band. But in first phase width and wavelength of photoluminescence band doesn’t change, photoluminescence intensity increases. So these changes are caused not by aggregation of quantum dots but by changes in quantum dot coating. CdTe-TGA quantum dots are fluorescent nanoparticles composed of CdTe core and TGA coating. Rearrangement of quantum dot B
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