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V. Poderys et al. / Medical Physics in the Baltic States 7 (2009) 24 - 29 spectra were measured with Varian Cary Eclipse (Varian Inc., Australia) and PerkinElmer LS 50B (PerkinElmer, USA) fluorimeters. Photoluminescence excitation wavelength was 405 nm, excitation slits were 5nm, and emission slits 5nm and 4 nm for Varian Cary Eclipse and PerkinElmer LS 50B, respectively. Measurements were done in 1 cm path length quartz cells (Hellma, Germany). Cells were hermetically sealed and between measurements kept in dark at room temperature. Photoluminescence decay was measured with FLS920 (Edinburgh instruments, UK) using time correlated single photon counting technique. EPL-405 (Edinburgh instruments, UK) laser, emission wavelength 405nm and pulse repetition rate 500 kHz was used for excitation. Photoluminescence decay was measured at peak of photoluminescence band (550 nm). Samples for atomic force microscopy measurements were prepared by casting a drop (40 μm) of solution on freshly cleaved V-1 grade muscovite mica (SPI supplies, USA) spinning at 1000 rpm. Commercially available atomic force microscope (AFM) diInnova (Veeco instruments inc., USA) was used to take 3dimensional (3-D) images of quantum dots. RTESP cantilevers (Veeco instruments inc., USA) with a tip curvature of >10 nm and a nominal spring constant of 40 Newton/meter (N/m) were used. Measurements were performed in tapping mode in air. All images were acquired at a scan rate of 1 Hz per line with a 512 pixel x 512 pixel image definition. Image processing included flattening (2nd order) to remove the background slope caused by the irregularities of the piezoelectric scanner. The analysis was performed using the SpmLabAnalysis software (Veeco instruments inc., USA). 3. Results We investigated dynamics of quantum dots spectral properties (photoluminescence intensity, position of photoluminescence maximum and width of photoluminescence band) in aqueous solutions. Dynamics of quantum dots photoluminescence properties are presented in Fig. 1. As can be seen from Fig.1 A photoluminescence intensity of quantum dots dissolved in deionized water increases from 556 a.u. just after preparation, till 937 a.u. after 88 hours. At this stage photoluminescence intensity increases to 168 % of initial value, but photoluminescence band maximum position and width at half maximum remains constant, respectively 548 nm and 53 nm. After 88 th hour photoluminescence intensity begins to decrease and decreases slowly till 142 nd hour (805 a.u.,143% of initial photoluminescence intensity). When photoluminescence intensity begins to decrease (after 88 hours) photoluminescence band (maximum at 548 nm) starts to shift to longer wavelength region and shifts by 26 nm to 574 nm(at 182 nd hour), but width of photoluminescence band still remains constant till 112 th hour. From 112 th hour photoluminescence band starts getting narrower and width of 25 photoluminescence band decreases from 53 nm (112 th hour) to 45 nm (142 nd hour) and later width of band remains constant. After 142 hours photoluminescence intensity starts to drop quite fast and 230 hours after preparation of solution photoluminescence disappears (intensity is 25 a.u.). Fig. 1 B shows absorbance band intensity dependence on time. Photoluminescence intensity, a.u. Absorbance, o.d.u. Photoluminescence intensity, a.u. Absorbance, o.d.u. 1000 750 500 250 Δλ FWHM , nm PL intensity 0 0 100 200 40 300 540 0.20 0.15 0.10 0.05 0.00 800 600 400 200 0 0.15 0.10 0.05 A 0.14 0 10 20 30 40 50 Time, h λ max Δλ FWHM 60 55 50 45 0 100 200 300 Time, h C 0.20 0.18 0.16 B PL intensity λ max Δλ FWHM 55 50 45 0 100 200 300 D Time, h Δλ FWHM ,nm 0.00 0 100 200 300 Time, h 580 570 560 550 580 570 560 550 λ max , nm λ max ,nm Fig. 1. CdTe QD spectral properties dependence on time. Photoluminescence band intensity, position and width dynamics: A - in deionized watter, C – in saline. Quantum dot
V. Poderys et al. / Medical Physics in the Baltic States 7 (2009) 24 - 29 absorption band intensity dynamics: B- in deionized water (insert shows initial absorption change 0-48 h), D – in saline, (concentration of quantum dots, c=3,84·10 -7 mol/l). Quantum dots absorbance is decreased after 24 hours but later it slowly increases till 192 nd hour. After 192 nd hour precipitate starts forming and absorbance intensity decreases. In case when saline is used instead of deionized water, one can see similar changes of CdTe quantum dots photoluminescence properties like described above. During first stage photoluminescence intensity increases, and photoluminescence band maximum position and width at half maximum remains constant, respectively 548 nm and 53 nm. At this stage photoluminescence intensity increases to 166 % of initial value (from 389 a.u. to 648 a.u.) at 144 th hour. Later photoluminescence intensity starts to decrease and 274 hours after preparation of solution is close to zero (40 a.u.). During this stage, when photoluminescence intensity is decreasing, photoluminescence band starts to shift bathochromically (163 th hour) and shifts by 25 nm from 550 nm to 575 nm (at 244 hour). Width of photoluminescence band starts to decrease 226 hours after solution preparation and narrows by 9 nm (from 54 nm to 45 nm at 253 hour). Absorbance band intensity dependence on time is presented in Fig. 1D. Quantum dots absorbance in saline remains constant till 192 nd hour. After 192 nd hour precipitate starts forming and absorbance intensity decreases (Fig. 1 D insert). All these changes are very similar to changes that appear when quantum dots are dissolved in deionized water. Intensity increase, shift of photoluminescence band and decrease of band width in both cases are very similar: intensity increases by 168% (deionized water) and 166% (saline), band shifts by 26 nm (deionized water) and 25 nm (saline), band narrows by 8 nm (deionized water) and 9 nm (saline). Despite all these similarities one difference can also be observed: phase of photoluminescence growth is longer in case of quantum dots in saline (144 hours compared to 88 hours). These results show that presence of Na + and Cl - ions doesn’t make big influence on spectral properties of CdTe quantum dots in aqueous media. In biological objects quantum dots are exposed to various ions and biomolecules. In circulatory system quantum dots stars to interact with serum proteins. Freshly prepared CdTe quantum dots solution (V=2 ml) was titrated with BSA solution (c=10 -4 mol/l). Fig. 2 shows, that adding BSA to CdTe quantum dots solution increases their photoluminescence intensity up to certain concentration of BSA. Saturation is achieved when concentration of protein is c=10 -5 mol/l. Further titration leads to photoluminescence decrease (Fig. 2 line A). Change of photoluminescence intensity is caused by two processes: interaction of quantum dots with proteins and dilution effect. Graph of reference solution (titrated with saline) is presented in Fig. 2 (line B). Difference between photoluminescence 26 intensity of solution titrated with BSA and photoluminescence of solution titrated with saline gives photoluminescence change caused by interaction between CdTe quantum dots and BSA (Fig.2 line C). This line reaches its maximum value at c=1.05·10 -5 mol/l. Photoluminescence Intensity, a.u. 900 800 700 600 + n* 20μl saline + n* 20μl BSA sollution Change of PL intensity 0.0 1.0x10 -5 Concentration of BSA, mol/l 2.0x10 -5 300 200 100 Fig. 2 CdTe quantum dots (CdTe c = 4,8 · 10 -7 mol/l, V = 2 ml) photo photoluminescence intensity: A – during titration with BSA solution (c = 10 -4 mol/l), B – titration with saline, C –change of photoluminescence intensity caused by BSA (dilution effect is eliminated). For further investigations we chose c = 1.5·10 -5 mol/l concentration of albumin. We decided to use 1.5 time bigger concentration, than concentration giving biggest photoluminescence increase, to ensure that all quantum dots are interacting with BSA. Photoluminescence spectral properties and absorption band intensity dynamics of CdTe quantum dots aqueous solutions with BSA is presented in Fig. 3. After the addition of protein photoluminescence of quantum dots suddenly increases (by 27% and 68% a.u. for saline and deionized water solutions, respectively). After jump, photoluminescence intensity further increases for approximately 40 hours and reaches 157% of initial value for saline and 184 % of initial value for deionized water solution. Later photoluminescence intensity starts decreasing, but decrease of intensity is quite slow and at longer time scale becomes negligible. This shows that BSA stabilizes CdTe quantum dots in aqueous media. Even after two months quantum dots remain in solution and precipitates don’t appear (Fig.3 D insert). Photoluminescence of quantum dots solutions with protein remains more intense than 50% of initial value. Absence of precipitate and quite intense photoluminescence of quantum dots solutions shows that BSA prevents quantum dots from aggregation. AFM measurements of long kept solutions confirm this idea. In Fig. 4 D CdTe quantum dots, deposited from saline solution after precipitate was formed, is presented. Large layered structures are seen on the surface. Height of these images is 70nm – 130 nm. Phase image (insert of Fig. 4 D) clearly shows that these large structures are formed from layers. Height of one layer is approximately equal to 3.5nm - 4 nm. Diameter of CdTe quantum dots that A C B 0 Change of Photoluminescence Intensity, a.u.
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