Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 126 (2014) 28–35

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Quantum dots as a possible oxygen sensor Paulina Ziółczyk, Katarzyna Kur-Kowalska, Małgorzata Przybyt, Ewa Miller ⇑ Institute of General Food Chemistry, Lodz University of Technology, Stefanowskiego 4/10, 90-924 Łódz´, Poland

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Fluorescence properties of ZnS:Cu

a r t i c l e

i n f o

Article history: Received 9 October 2013 Received in revised form 15 January 2014 Accepted 22 January 2014 Available online 7 February 2014 Keywords: Quantum dots Oxygen sensor Fluorescence Quenching

Io /I 2.1

3

Fluorescence intensity [a.u.]

quantum dots depend on amount of added copper.  Fluorescence intensity of ZnS:Cu quantum dots is increasing with increasing pH.  Elimination of oxygen increases fluorescence of quantum dots 3–4 times.  Fluorescence of ZnS:Cu quantum dots is quenched by oxygen.  Quenching constants varies from 2.7 to 4.6 mM1 depending on copper addition and pH.

without O2

1.7 2

1.5

1.5

1.3 solution saturated

0.9

with O2 from air

0.5

0.7 0.5

0 400

450

500

λ [nm]

550

0

0.05

0.1

0.15

0.2

0.25

0.3

c oxygen [mM]

a b s t r a c t Results of studies on optical properties of low toxicity quantum dots (QDs) obtained from copper doped zinc sulfate are discussed in the paper. The effect of copper admixture concentration and solution pH on the fluorescence emission intensity of QDs was investigated. Quenching of QDs fluorescence by oxygen was reported and removal of the oxygen from the environment by two methods was described. In the chemical method oxygen was eliminated by adding sodium sulfite, in the other method oxygen was removed from the solution using nitrogen gas. For elimination of oxygen by purging the solution with nitrogen the increase of fluorescence intensity with decreasing oxygen concentration obeyed Stern–Volmer equation indicating quenching. For the chemical method Stern–Volmer equation was not fulfilled. The fluorescence decays lifetimes were determined and the increase of mean lifetimes at the absence of oxygen support hypothesis that QDs fluorescence is quenched by oxygen. Ó 2014 Elsevier B.V. All rights reserved.

Development of biotechnology and biomedicine requires still new, quick, sensitive and selective analytical and diagnostic methods. Hence, optical biosensors due to their interesting properties have become a subject of studies in many research centers in the world. Particularly noteworthy are the sensors with the use of fluorescence techniques because of their very high sensitivity and selectivity. The essence of this type of sensors is an immobilization in the sensor layer fluorophores whose emission ⇑ Corresponding author. Tel.: +48 42 631 34 26; fax: +48 42 636 28 60.

http://dx.doi.org/10.1016/j.saa.2014.01.112 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

y = 3.7369x + 1 r2 = 0.9924

1.1 1

Introduction

E-mail address: [email protected] (E. Miller).

1.9

2.5

is sensitive to the presence of a determined reagent or to a change in the tested system properties [1–4]. Oxygen is a very important element involved in biological and chemical processes, so there is special group of sensors based on photoluminescence, which enable determination of its concentration [5–7]. In many reactions catalyzed by enzymes, oxygen is also one of substrates. Due to this, by determining O2 concentration it is also possible to determine indirectly the presence and concentration of many biologically significant substrates, e.g. glucose, in the reaction of its oxidation by glucose oxidase [8,9]. For several years, there has been growing interest in quantum dots (QDs) because of their unique optical properties caused by quantum confinement [10,11]. Quantum dots are semiconductor

P. Ziółczyk et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 126 (2014) 28–35

nanocrystals of dimensions ranging from several do tenths of nanometres. They are composed of elements from groups II–VI or III–V. Their photoluminescence depends on their dimensions and the maximum of it is growing with the increasing radius. QDs have interesting optical properties such as broad excitation band and narrow emission band and are characterized by high Stokes shift. Additionally, good photostability of quantum dots and their resistance to metabolic degradation and photobleaching makes them applicable in bioanalysis [12–15]. The use of quantum dots and their conjugates with enzymes enables the observation of biochemical reactions on the cell level in living organisms in the time range from a second to over several days [16]. Moreover, quantum dots can find application in protein detection, DNA sequencing and in immunological tests. QDs play also an important role in testing new medicines, allowing researchers to follow the way in which their particles are transported to a relevant receptor [16,17]. The core of quantum dots is usually composed of elements belonging to groups II and VI, e.g. toxic quantum CdSe dots [12] or low-toxic ZnS ones [11]. Contrary to cadmium selenide, which due to the toxicity of cadmium cannot be applied in vivo determination, a promising material is just quantum ZnS dots. Interesting and relatively poorly known are optical properties of these luminophores doped with various transient metal ions such as Mn2+ [18,19], Cu2+ and Cu+ [20,21], Ag+ [22] and Co2+ [23]. The presence of these metals causes a modification of spectral properties of quantum dots. Hence, they have become the subject of studies in many research centers in the world. The most often used dopants are Mn2+ ions [14,24,25]. Martínez-Castañón et al. [24] described a simple method of synthesis and optical characteristic of ZnS, ZnS:Mn particles and their equivalents with CdS coating. The ZnS:Mn quantum dots obtained by them showed the fluorescence emission with a maximum shifted towards longer wavelengths (580 nm) in relation to pure ZnS quantum dots. The ZnS:Mn dots with CdS coating synthesized by them were characterized by an increased fluorescence emission at 580 nm as compared to quantum dots without coating. On the other hand, Khosravi et al. [25] described the maximum emission of ZnS quantum dots doped with Mn2+ ions at 600 nm. Khani et al. [14] studied the effect of Fe3+ ions on the optical properties of ZnS quantum dots. They found that this addition caused a shift of the fluorescence emission maximum from 427 nm for pure ZnS QDs to 442 nm for doped one. Addition of Fe3+ was most probably responsible for the appearance of a next emission band at 532 nm. Optical properties of quantum dots doped with copper ions are poorly known. The mechanism of their photoemission has not been fully explained. Doping of quantum dots with copper ions causes an increase of fluorescence intensity or its quenching, depending on their concentration [26]. Manzoor et al. [27] found that for ZnS quantum dots the addition of copper only caused a decrease of photoluminescence intensity, however once a co-activator in the form of halide (e.g. F ions) was added, the fluorescence emission increased. The addition of copper ions causes a shift of the emission maximum of ZnS dots towards longer wavelengths [27,28]. A separate class of studies are the experiments carried out not with solutions but with the use of dry quantum dots. There are reports on the shift of emission bands in a broad range from 466 nm [29] through 490–510 nm [30] to as much as 600 nm [31]. Zheng et al. [32] observed two emission bands for ZnS:Cu, the first one with a maximum at 450 nm and the second one at 526 nm. Similarly, Yang et al. [33] who studied the properties of ZnS quantum dots doped with copper, lead and both of them, reported two emission bands for ZnS:Cu at 450 nm and 530 nm. The aim of presented paper is the study of factors influencing ZnS quantum dots fluorescence from the point of view of their

29

further application in biosensors. Results of the steady-state and time-resolved fluorescence measurements for synthesized ZnS quantum dots doped with various amounts of copper are presented and discussed. Also the influence of oxygen presence in solution on QDs fluorescence was studied and is reported.

Experimental Materials ZnSO47H2O (P99.0%) was purchased from Sigma Aldrich (Germany), CuSO45H2O, Na2S9H2O and Na2SO3 were purchased from POCH S.A. (Poland), mercaptopropionic acid (MPA) (P99.0%) was purchased from Fluka (Germany). Distilled water was used throughout.

Synthesis of Cu-doped quantum dots ZnS:Cu quantum dots were prepared according to [22] with slight modifications. Nanoparticles were synthesized with different CuSO4 to ZnSO4 volume ratios: 1:49 (A), 2:48 (B) and 3:47 (C). For example for Cu:Zn ratio 1:49 (sample A), 4.9 mL of 0.1 M ZnSO4 solution was mixed with 0.1 mL of 0.1 M CuSO4 and 0.17 mL of MPA, added with water to obtain the final volume of 50 mL and adjusted to pH 11.5. Then 5 mL of 0.1 M Na2S was added and this mixture was heated for 30 min at 95 °C. Next, the mixture was cooled to room temperature and QDs were precipitated by adding 75 mL of ethanol. QDs were harvested by centrifugation, washed with ethanol and dried overnight at 40 °C. The stock solution of QDs was prepared as follows: 10 mg of ZnS:Cu QDs were dissolved in 2 mL of 0.01 M phosphate buffer, pH 7. For measurements 100 lL of stock solution was added to a 10 mL flask and dissolved with buffer giving final concentration of QDs 0.05 mg/mL.

Apparatus and measurements Absorbance spectra were recorded using a spectrophometer Nicolet Evolution 300 (Thermo Scientific, USA) in 10 mm path length quartz cells. Steady-state fluorescence measurements were performed using a Fluoromax-4 spectrofluorometer (Jobin Yvon-Spex Instruments S.A., Edison, New Jersey, USA). The fluorescence spectra were measured with 10 mm path length closed quartz cells. The excitation and emission slits were set at 5 nm each. The increment was set at 1 nm and integration time at 0.5 s. The measurements were carried out at ambient room temperature. Oxygen concentration in solutions expressed as % of saturation with O2 from air was measured using a galvanic silver–zinc oxygen electrode CTN-920.S (MES-EKO, Wrocław, Poland) connected with a CO-551 oxygen meter (Elmetron, Zabrze, Poland). Fluorescence emission decays were measured with a timecorrelated single photon counting apparatus from the Edinburgh Instruments Co (UK), equipped with a pulsed NanoLED diode (Horiba JobinYvon IBH Ltd., UK) as an excitation light source. The used diode had peak wavelength 395 nm and pulse duration less than 0.01 ns. The measurements were carried out with the emission monitored at a 90° angle to the excitation. The instrument profile was obtained by replacing the sample with Ludox as a scatter. The data were collected in 1023 channels to 10,000 counts in the peak, and the time calibration was 53 ps per channel. The data were analyzed by a tail fit procedure [34] using the software package provided by the Edinburgh Instruments.

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P. Ziółczyk et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 126 (2014) 28–35

The microstructure and dimension of the synthesized QDs particles were determined by a TESLA BS-500n (Brno, Czech Republic) transmission electron microscope (TEM). Quenching experiments with N2 Briefly, 5 mg of QDs were dissolved in 100 mL of 0.01 M phosphate buffer, pH 6 or 7. The obtained solution was aerated during 15 min by intense mixing. Then the vessel with the solution was closed and purged slowly with N2 under continuous mixing. Oxygen electrode connected with oxygenmeter was dipped in the solution and the concentration of O2 expressed as % of saturation from air was read. At the beginning and for every 10% decrease of saturation level 3 mL of solution were aspired by a syringe, poured quickly into quartz cell and the fluorescence was measured immediately. Fluorimetric titration of quatum dots solution with Na2SO3 3 mL of QDs solution of concentration 0.05 mg/mL saturated with oxygen from air were poured into closed quartz cuvette and fluorescence was measured. Then 3 lL of 0.05 M Na2SO3 solution was added with Hamilton syringe. Solution in the cuvette was carefully mixed and the measurement of fluorescence was repeated. This procedure was repeated 11 times until the total volume of added Na2SO3 solution reached 33 lL. Results and discussion Optical properties of the obtained quantum dots Fig. 1(a) shows absorbance spectra of a freshly prepared solution of Cu-doped ZnS QDs. Synthesized quantum dots containing different amounts of copper were used in the tests. Thus, in QDs obtained by procedure A, the content of copper is the smallest, while in dots formed in process C it is the highest. The absorption band can be observed as a shoulder at 297 nm for each of the three concentrations of copper. For the quantum dots, the position of absorption band edge (E*) could be taken as a measure of their size [35,36]. The radius of particle can be calculated from the absorbance spectra, using Brus equation (1):

E  Eg þ

h2 p2 2R2



 1 1 1:8e2  þ me mh 4pe0 er R

ð1Þ

Eg is the band gap energy of bulk semiconductor (eV); mh the effective mass of holes; me the effective mass of the electrons; R the particle size, (nm); ⁄ the Planck constant divided by 2p (J s); e the relative permittivity and e0 the permittivity of vacuum and e is the charge on the electron. Values of E* of synthesied QDs were found from Tauc plots (Fig. 1b) [35]. Their values for all studied QDs were very similar – about 4 eV giving the diameter of nano particles about 4.8 nm. The sizes and optical properties are summarized in Table 1. The TEM micrograph in Fig. 2 shows that the distribution of diameter of QDS is from 6 nm to 20 nm, which is rather in poor agreement with the result from the absorption spectra. The observed difference in sizes obtained by both methods is a result from the fact that by the absorbance only the core of the nanoparticle is observed [37,38]. The fluorescence spectra were obtained using an excitation wavelength 297 nm. The concentration of solution was 0.05 mg/ mL and the absorbance of them at 297 nm were nearly the same as can be seen from Fig. 1a. The maximum of fluorescence emission was observed at about 460–470 nm varying slightly with sample type and pH of solution. The results are similar to those reported by other authors [32,39]. Fig. 3 shows example of fluorescence emission spectra of three different samples of obtained QDs (A, B, C). One can see that the lower amount of copper ions was added the higher is the fluorescence. With increasing amount of copper ions the position of fluorescence maximum is slightly shifted to longer values by about 10–15 nm. The fluorescence characteristics of studied QDs is summarised in Table 2. The emission spectra are very broad but are narrowing with increased amount of copper. It can be seen that fluorescence intensity is reduced by the increasing addition of Cu, which could be a result of the binding of Cu2+ ions to the surface of QDs [26,30,40,41]. Kole et al. [30] and Peng et al.

Table 1 Spectral feature, optical band gap and size of obtained quantum dots. Sample type

V CuSO4 (mL): V ZnSO4 (mL)

E* (eV)

keg (nm)

R (nm)

D (nm)

A B C

1:49 2:48 3:47

4.029 4.025 4.035

307.8 308.1 307.3

2.42 2.43 2.40

4.84 4.86 4.80

E* (eV) – the band gap energy of synthesized QDs. keg (nm) – the absorption onset. R (nm) – the particle radius. D (nm) – the particle diameter.

(a)

(b)

6

2

A B C

5

B

1.5

C 1

(hνA)2 [(eV)2]

Absorbance

A 4

3

2 0.5 1

0 200

0 250

300

λ [nm]

350

3.9

4.1

4.3 hν [eV]

4.5

Fig. 1. (a) Absorbance spectra of ZnS:Cu quantum dots solution, pH 7, concentration 0.05 mg/mL. (b) Tauc plots for the determination of optical band gap [35] (A, B, C – type of synthesed QDs).

P. Ziółczyk et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 126 (2014) 28–35

31

Fig. 2. Representative TEM images of ZnS:Cu quantum dots (sample A).

and C about 10 nm shift of the maximum towards longer wavelengths can be observed.

Fluorescence intensity [a.u.]

2 1.8

A

1.6

B

Influence of pH and oxygen on QDs fluorescence

C

1.4 1.2 1 0.8 0.6 0.4 0.2 0 390

410

430

450

470

490

510

530

550

[nm] Fig. 3. Fluorescence emission spectra of QDs; kexc = 297 nm (A, B, C – type of synthesed QDs).

Table 2 Fluorescence characteristics of quantum dots solutions kexc = 297 nm. Buffer pH

Sample type

kmax (nm)

Emax (eV)

W1/2 (eV)

6

A B C

464 475 474

2.67 2.61 2.62

0.634 0.582 0.563

7

A B C

461 473 471

2.69 2.62 2.63

0.652 0.572 0.576

kmax – position of fluorescence emission maximum. Emax – energy at fluorescence emission maximum. W1/2 – half width of emission band.

[26] suggest that at a higher copper concentration in ZnS quantum dots most probably CuS is formed and so the number of Cu2+ ions which can play the role as active luminescence centers is reduced. Yang et al. [42] described quenching of ZnS:Cu QDs fluorescence by the excess Cu2+ and Cu+ ions. According to the authors, in both cases the mechanism of photoluminescence is different. Xue et al. [41] reported that Cu2+ dopant ions had a significant effect on ZnSe QDs fluorescence and that above some concentration of Cu the photoluminescence of synthesized quantum dots at 400 nm disappeared completely. Table 2 shows a comparison of kmax and W1/2 ZnS:Cu nanoparticles at pHs 6 and 7. There are only subtle differences between pHs, but in both cases for samples B

Preliminary experiments with ZnS:Cu QDs and glucose oxidase (not reported here) indicate that QDs fluorescence is influenced by presence of oxygen. At the presence of glucose oxidase and excess of glucose in the solution when oxygen is totally consumed in the enzyme reaction the fluorescence of ZnS:Cu Qds increased dramatically. This observation was inspiration and indications to examine in detail the influence of molecular oxygen on photoluminescence of quantum dots. So, while studying the effect of pH on fluorescence emission of quantum dots, measurements were made for both aerated solutions and these with no O2 content. Oxygen was eliminated by addition of the excess of Na2SO3, which reacted with oxygen according to the formula:

2Na2 SO3 þ O2 ! 2Na2 SO4

ð2Þ

To assure total elimination of O2 30 lL of 1 M Na2SO3 solution were added to 3 mL of QDs solution. Fig. 4 shows fluorescence emission spectra of QDs at different pHs ranging from 5 to 10 at the presence and absence of oxygen. For QDs solutions saturated with oxygen from air fluorescence intensity is growing with increasing pH, wherein the greatest increase is observed when pH is changing from 5 to 6. This effect may be probably connected with the change of the buffer type or more probably by dissociation of surface coating MPA. At low pH the carboxylic group is not dissociated and at higher one dissociates into –COO increasing the surface charge of QDs. In the acid environment fluorescence of QDs was the lowest which suggested quenching of the emission intensity by high proton concentration. Supposedly, they can interact with sulfur or hydroxyl groups on the surface of QDs [43]. For deoxygenated solution the highest fluorescence intensity was observed at pHs 6 and 7 then it is decreasing with increasing pH. After the elimination of oxygen, the fluorescence intensity increased several times, with the shift of maximum to longer wavelengths by about 10 nm. Fig. 5 shows the ratio of the total fluorescence intensity at the absence and at the presence of oxygen at different pHs for three different dopant additions. To obtain the total fluorescence the emission spectra were integrated in the range from 400 nm to 550 nm. It could be seen that at pHs 5, 6 and 7 these ratios were the highest. These results indicate that the fluorescence of ZnS:Cu quantum dots is strongly influenced (quenched) by oxygen. For further detailed study of oxygen influence pHs 6 and 7 were chosen because for

32

P. Ziółczyk et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 126 (2014) 28–35

9

pH 5 pH 6 pH 7 pH 8 pH 9 pH 10

Fluorescence intensity [a.u.]

8 7 6 5

I0 ¼ 1 þ K SV ½Q  I

4 3

(a)

2 1 (b)

0 380

400

420

440

460

480

500

520

540

[nm] Fig. 4. Fluorescence emission spectra of sample A Cu-doped QDs solution at different pHs at the presence and absence of oxygen, kex = at 297 nm, concentration 0.05 mg/mL; (a) – at the absence of O2 in the solution, (b) – solution saturated with O2 from air.

I at absence of O2/I at presence of O2 from air 6.0 A B C

5.0 4.0 3.0 2.0 1.0

pH 0.0 4

der at 325 nm was found, fluorescence measurements were made for these two wavelengths. The obtained emission spectra were integrated between 400 nm and 550 nm to calculate total fluorescence. Generally quenching of fluorescence is described by the Stern– Volmer equation:

5

6

7

8

9

10

11

Fig. 5. Ratio of total fluorescence emission of QDs solution at the absence of O2 and saturated with O2 from air as a function of pH, kexc = 297 nm (A, B, C – type of synthesed QDs).

them the increase of fluorescence was the highest and they are similar to optimum of many enzymes and natural environment in living cells. Quenching of QDs fluorescence by oxygen The previously obtained results have become a basis of more precise studies of influence of oxygen on fluorescence of ZnS:Cu quantum dots. For this purpose the steady-state measurements of fluorescence were carried out for QDs solutions in phosphate buffers containing oxygen and after its gradual elimination. Removal of oxygen from the solution could be achieved by purging it with an inert gas (N2 or Ar). An alternative method is the addition of a reducing agent, which reacts with oxygen as Na2SO3 as it is indicated by the reaction (2) [44,45]. Both methods were applied to investigate the influence of oxygen on QDs fluorescence. Fig. 6 shows an example of results obtained by purging QDs solution with N2 for sample B at pH 6. One could see that the fluorescence increased several times and the emission maximum was shifted about 10 nm towards longer wavelengths when oxygen concentration was tending to zero. Similar results were obtained for all samples of QDs at both studied pHs e.g. 6 and 7. Oxygen concentration in the solution saturated with air at atmospheric pressure and temperature 25 °C is about 0.256 mM. Knowing the level of saturation from measurements with oxygenmeter, oxygen concentrations in solution were calculated. As in fluorescence excitation spectra (not shown) beside the main maximum at 297 nm, a shoul-

ð3Þ

I0 is the fluorescence intensity at the absence of a quencher, I the fluorescence intensity at the presence of a quencher, [Q] the quencher concentration (mM) and KSV the Stern–Volmer quenching constant (mM1). The values of total fluorescence were presented in Stern– Volmer plot (insert on Fig. 6). One could see that a very good agreement with assumed mathematical model was achieved. Values of calculated KSV are collected in Table 3. In all cases values obtained at pH 7 are slightly higher than at pH 6. For each sample values obtained for both excitation wavelengths are nearly the same with some exception for sample A at pH 6. The amount of added Cu2+ ions during QDs synthesis has some influence on obtained results but it is hardy to find general correlation. At pH 6 the greatest values of KSV were found for sample B. At pH 7 constants are for samples A and B are practically the same and the lowest for the highest amount of Cu (sample C). Generally the differences are not very significant. There is very little information in literature about influence of oxygen on QDs fluorescence in solution. Wang and co-workers [5] described a visible light induced photoelectrochemical biosensor for the detection of glucose based on oxygen-sensitive NIR nanoparticles. The consumption of oxygen during enzymatic reaction was used to measure the substrate. Authors claimed that increase of current under illumination indirectly indicated interactions of oxygen with NIR nanoparticles. Xia et al. [46] reported reversible quenching of CdTe quantum dots fluorescence by oxygen. Particle size, capping ligands and pH were proven to govern the quenching process. The quenching of QDs fluorescence by oxygen is most probably a result of interaction of O2 molecules with a surface of QDs. Absorption of UV light causes the jump of electron from valence band to conduction band of QDs with hole formation in valence band. Additional electrons introduced to the conduction band by oxygen increased the rate of electron–hole recombination process, which is non-radiative one. As the effect oxygen elimination from the solution causes the increase of fluorescence. In a similar way, Sung et al. [47] explained an increase of the intensity of fluorescence of ZnO/ZnS-MAA-GOx bioconjugates (dots with ZnO/ZnS modified by mercaptoacetic acid (MAA) and glucose oxidase (GOx). Additionally O2 molecules can react with electrons in conduction band causing decrease of their amount [49]. The results of experiments on oxygen influence on QDs fluorescence with chemical elimination of O2 are at first glance similar to the former ones. Example of such results is presented on Fig. 7 for sample A at pH 6. In the case of oxygen removal by adding sodium sulfite, it was observed, like in the case of the method with the use of nitrogen, that fluorescence intensity increased several times with a simultaneous slight shift of the emission maximum position towards longer wavelengths. But the attempt to fit the data to the Stern–Volmer dependence gave no results (insert on Fig. 7). One can see strong deviations from linearity, the dependence of I0/I on oxygen concentration is overlinear. Because of that, there is impossible to find KSV. In this case, when oxygen is eliminated by chemical method, the by-products of reaction (2) can affect also the fluorescence of QDs. To determine precisely the effect of oxygen on QDs fluorescence emission the time-resolved measurements were also carried out. It

33

P. Ziółczyk et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 126 (2014) 28–35 Io/I 2.1 1.9

3

1.7 1.5

0% 2.5

Fluorescence intensity [a.u.]

y = 3.7369x + 1

1.3

2

r = 0.9924

1.1 0.9 0.7 0.5

2

0

0.05

0.1

0.15

0.2

0.25

0.3

c oxygen [mM]

1.5

100 %

1

0.5

0 400

420

440

460

480

500

520

540

λ [nm] Fig. 6. Fluorescence emission spectra of QDs solution (sample B, pH 6) for different levels of saturation with O2 from air, kexc = 297 nm; insert: plot of I0/I as a function of oxygen concentration (I0 – total emission at the absence of O2 and I – total emission at the presence of O2).

Table 3 Constants for quantum dots fluorescence quenching by oxygen obtained by controlled purging with N2. Sample type

kex (nm)

K (mM1) pH 6

pH 7

A

297 325 297 325 297 325

2.71 ± 0.09 3.06 ± 0.10 3.74 ± 0.05 3.65 ± 0.04 2.97 ± 0.08 3.00 ± 0.09

4.47 ± 0.11 4.59 ± 0.11 4.39 ± 0.09 4.65 ± 0.13 3.55 ± 0.13 3.62 ± 0.15

B C

was possible to determine the lifetimes only for quantum dots, which contained the smallest copper content (sample A) due to limitations of the apparatus that made it impossible to record the spectra of dots with lower emission. Examples of decay at pH 7 for QDs at the presence of oxygen and after it elimination by both methods are shown on Fig. 8. The obtained kinetics of photolumi-

nescence decay was described by a tri-exponential function and the results are given in Table 4. When analyzing the data from Table 4, one can observe that the first two decay times s1 and s2 after oxygen removal remain practically unchanged. The first life time is about 4.5 ns, the second one is between 25 and 31 ns. However, the third time s3 after O2 elimination is significantly growing and simultaneously its contribution (a3) grows also. As the result the mean lifetime is greater at the absence of oxygen as compared with the one obtained for solution saturated wirh oxygen from air. There is also significant difference between results obtained for different methods of oxygen elimination. For the chemical method the increase of mean lifetime is less as compared with the physical one. There is very little data in literature about fluorescence lifetimes of ZnS QDs. Khosravi et al. [48] identified lifetimes for synthesized ZnS dots and copper-doped ZnS dots. In both cases the kinetics was bi-exponential and the determined lifetimes were 1.62 ns and 22.12 ns for ZnS QDs, and 2.9 ns and 54.2 ns for Cu-doped ZnS QDs. These times are roughly similar to the first two times obtained in this work. On the other hand, Bol et al. [31] checked Io/I 3.5 3.0 2.5

6.0

0 mM

Fluorescence intensity [a.u.]

2.0

5.0

1.5 1.0

4.0

0.5 0

0.05

0.1

0.15

0.2

0.25

0.3

c oxyge n [mM]

3.0 2.0

0.256 mM 1.0 0.0 400

420

440

460

480

500

520

540

λ [nm] Fig. 7. Fluorescence emission spectra of QDs solution (sample A, pH 6) for decreasing concentration of O2 controlled by addition of Na2SO3, kex = 297 nm; insert: plot of I0/I as a function of oxygen concentration (I0 – total emission at the absence of O2 and I – total emission at the presence of O2).

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P. Ziółczyk et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 126 (2014) 28–35

10000

Counts

(a)

(b)

(c)

1000

100 10

60

110

160

210

260

310

360

410

460

Time [ns] Fig. 8. Fluorescence decays of ZnS Cu-doped QDs in solution (pH 7); (a) saturated with O2 from air, (b) deoxygenated with Na2SO3, and (c) deoxygenated by purging with N2.

Table 4 Lifetimes of A QDs photoluminescence at the absence and presence of oxygen. pH

Conditions

s1 (ns)

a1

s2 (ns)

a2

s3 (ns)

a3

hsi (ns)

v2

6

Presence of oxygen Na2SO3 N2

4.29 ± 0.14 4.63 ± 0.15 4.67 ± 0.28

0.09 0.08 0.04

24.9 ± 0.9 27.6 ± 1.0 25.9 ± 1.2

0.25 0.22 0.18

130.0 ± 3.1 161.2 ± 4.8 163.6 ± 5.0

0.67 0.70 0.77

93.1 119.8 131.5

1.07 1.21 1.16

7

Presence of oxygen Na2SO3 N2

4.79 ± 0.16 4.36 ± 0.17 4.94 ± 0.14

0.10 0.07 0.08

27.0 ± 1.0 25.5 ± 0.9 31.4 ± 1.9

0.26 0.23 0.16

130.7 ± 3.4 139.6 ± 3.3 191.7 ± 9.4

0.64 0.70 0.76

91.9 104.2 151.7

1.08 1.08 1.12

si – Lifetime. ai – Contribution of a lifetime. hsi – Mean lifetime calculated as hsi = Rai  si.

the effect of temperature on the lifetimes of quantum dots with ZnS:Cu. They observed a reduction of the lifetimes of dots from 20 ls at the temperature 4 K to only 0.5 ls at 300 K. The last results could be hardly compared with ours and Khosravi et al. [48] hence they were obtained for dry QDs. Devi and Negi [50] reported for pure ZnS capped by histidine nanoparticles triexponential decays as in our result but with lower values. None data about influence of oxygen on QDs photoluminescence lifetimes were found. The sensitivity of QDs photoluminescence emission to the presence of oxygen could be a basis to form an oxygen sensor and in conjunction with oxygen depended oxido-reductases (like glucose oxidase) to obtain a biosensor for enzyme substrate. Conclusions ZnS quantum dots doped with Cu2+ were obtained using quick and simple method of synthesis with different amounts of the dopant. The increase of doped Cu amount caused the decrease of fluorescence emission intensity with a shift of maximum emission to towards longer wavelengths. Fluorescence emission intensity of studied quantum dots depends on pH. With increasing pH, intensity of fluorescence is slightly growing. It was found that fluorescence intensity is quenched by oxygen. After elimination of oxygen by the addition the excess of Na2SO3, which reacts with oxygen, the fluorescence emission intensity is growing several times. This enhancement of fluorescence after elimination of oxygen depends also on pH and is the greatest for pHs 6 and 7 (about four time increase). At higher pH values, the effect of fluorescence

enhancement by oxygen elimination is decreasing with increasing pH. Therefore, the detailed studies of oxygen influence on QDs fluorescence were made for pHs 6 and 7. Two methods of gradual oxygen elimination from solution were applied: chemical (using oxygen-consuming Na2SO3) and physical (by purging solution with nitrogen). Results obtained by both methods were somehow different. For the physical method, the total fluorescence obeyed Stern–Volmer equation, for the chemical one not. The values of Stern–Volmer constants obtained in physical method are similar but it seems that they depend slightly on amount of added copper and pH. At pH 6 KSV are lower as compared with pH 7. Time-resolved measurements done for the QDs with the lowest addition of copper confirmed that fluorescence of QDs is influenced by oxygen. Fluorescence intensity decays were tri-exponential with the shortest lifetime about 4.5 ns, intermediate 27 ns. These both lifetimes do not depend practically on oxygen presence. The third longest one depends in contrary strongly on the presence of oxygen. It is growing when the oxygen is eliminated from the solution and also its contribution is increasing causing the increase of the mean lifetime. The values of mean lifetimes depend also on the method of solution deoxygenation. They are greater when oxygen is eliminated by physical method as compared with chemical one. The influence of oxygen on quantum dots fluorescence is most probably caused by interaction of oxygen molecules with QDs surface, which increases the rate of non-radiative electron–hole recombination. In case of chemical method of deoxygenation with the use of sodium sulfite also the by-products of its reaction with oxygen can influence the photoluminescence of QDs.

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