UV induced synthesis of starch capped CdSe quantum dots: Functionalization with thiourea and application in sensing heavy metals ions in aqueous solution

2.1 Materials

High purity chemicals CdSO_4, Na₂SO₃, Se powder, 2-propanol, acetone, and starch were obtained from sigma and used without further purification. Ammoniated cadmium sulphate, Cd(NH₃)₄SO₄ was used as the Cd precursor and freshly prepared sodium selenosulphate, Na₂SeSO₃ was used as the Se precursor. Sodium selenosulfate was synthesized by following the reported literature; typically, a mixture of 5 g of Na₂SO₃ and 0.5 g of Se powder was refluxed at 70 °C for 7 h to obtain a clear transparent solution, which contains Na₂SeSO₃ (250 mM) and unreacted Na₂SO₃ (Singh et al., 2011a,b). It is to be noted here that the excess Na₂SO₃ present in the solution neither take part nor hinder the reactions during the formation of CdSe QDs (Singh et al., 2011a,b). Zn(SO₄)·7H₂O, HgCl₂, Hg₂Cl₂, NiSO₄·6H₂O, MnSO₄·7H₂O, CoSO₄·7H₂O, CrCl₃, K₂CrO₄, CuCl₂, FeCl₃, PbCl₂ salts were used in the metal ion sensing experiments. Nanopure water obtained from a Millipore water purifying system (resistivity >18 mega ohm cm) was used for preparation of solutions.

2.2 Synthesis of CdSe quantum dots

5 mg/mL of potato starch solution was prepared by taking the required amount of potato starch in 25 mL of water and heated to 60^οC for 20 min. The initial turbid solution of starch became clear after heating. This process is termed as gelatinization of starch (Jenkins and Donald, 1998). Reaction mixture with different amount of CdSO₄ and Na₂SeSO₃ were taken in 10 mL flask containing 0.5 mg/mL of starch solution. 2% v/v of acetone and 2-propanol were added to it. The reaction mixture was irradiated in a quartz cell sealed with rubber septum, inside a Rayonet Photoreacter equipped with sixteen 300 nm UV lamps.

The concentrations of CdSO₄ and Na₂SeSO₃ were different in the case of different stoichiometric ratios, (i) 0.5:0.5 mM, (ii) 0.5:1.0 mM, (iii) 1.0:0.5 mM, (iv) 1.5:0.5 mM and (v) 2.0:0.5 mM. QDs extracted from the colloidal solution synthesized with 1.5:0.5 mM of precursors were functionalized with thiourea by taking 20 mg of QDs with 10 mM of thiourea in 10 mL water and refluxing it at 85 °C for 7 h. Metal ion sensing experiments were performed by diluting ten times the stock solution of the as functionalized QDs followed by addition of 10 mM of different metal ions. Interference study was performed by adding 0.1 mM of Cu²⁺, 1.0 mM of Cr⁶⁺ and 2.0 mM of Hg²⁺ along with equal and tem times higher concentration of interfering metal ions.

2.3 Characterization

UV–visible absorption spectra were recorded using spectrophotometer (Jasco V-650). Steady state PL spectra were recorded at room temperature using a spectrofluorometer (Hitachi F-450). PL lifetime measurements were carried out by time-correlated single photon counting (ICSPC) technique using a time resolved spectrofluorometer (Horiba Jobin Yvon IBH 400). X-Ray diffraction (XRD) measurements were carried out using panalytic (X-Pert pro) instrument. Raman spectral studies were carried out using Raman spectrometer (Seki’s STR300). FTIR measurements were carried out with the sample on KBr pallet using SHIMADZU IR- Affinity-1 FTIR instrument in diamond ATR mode. TEM images were recorded using a Libra 200FE instrument. SEM images were recorded using JEOL JSM-T330 SEM instrument.

3.1 Synthesis of starch capped CdSe QDs

The reaction mixture in one of the cases was prepared by taking 0.5 mM of CdSO₄, 0.5 mM of Na₂SeSO₃, 0.5 mg/mL of starch solution, 2% v/v of acetone and 2% v/v of 2-propanol. After photoirradiation, the colourless solution changed to orange coloured colloidal solution (Fig. 1). Acetone absorbs UV photons and in the excited state it abstracts an H atom from 2-propanol molecule to form two 2-hydroxy-2-propyl radicals, (CH₃)₂C^•OH which are highly reducing in nature (E⁰ = −1.5 V vs NHE) (Wardman, 1989). These radicals reduce Cd²⁺ and release Se⁻ which eventually react together to form CdSe QDs. The complete reaction mechanism is discussed below:

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Fig. 1

(a) Unirradiated reaction mixture containing Cd:Se precursors as 0.5:0.5 mM along with 0.5 mg/ml starch in the presence of 2% (v/v) acetone and 2-propanol, (b) orange-yellow coloured colloidal solution obtained after photoirradiation with 300 nm UV lamp and (c) camera-ready picture of the same colloidal solution under UV light.

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The growth of CdSe QDs was monitored by recording the absorption spectra at different time of irradiation. Fig. 2 shows the absorption spectra of a solution containing 1.5 mM of CdSO₄ and 0.5 mM of Na₂SeSO₃at different time of photoirradiation (For other Cd:Se ratios see the supporting information Figs. S1–S4). After around 10 min of photoirradiation, the absorbance did not increase further indicating the synthesis of CdSe QDs has been completed. In the absence of acetone, the colour of the reaction mixture does not change, which indicates the reaction is initiated by the excited state of acetone only. The excitonic peak position as well as particle size (calculated from Brus equation) were plotted at different time of photoirradiation (insets of Fig. 2). The PL spectra of as synthesized QDs (with different stoichiometric ratio of CdSO₄ and Na₂SeSO₃) were recorded (Fig. 3a).

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Fig. 2

Absorption spectra of reaction mixture containing Cd and Se precursor with starch at different time of photoirradiation (inset1 excitonic peak position and inset 2 particle size vs. time).

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Fig. 3

(a) steady-state PL spectra and (b) time-resolved PL decay profiles of CdSe QDs synthesized with different Cd:Se precursor ratios.

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It was found that the emission intensity increases with increase in Cd:Se ratio and was maximum for 1.5:0.5 mM. The QDs synthesized with Cd:Se ratio 0.5:0.5 mM and 0.5:1.0 mM were found to be non fluorescent in nature. The PL lifetimes of the CdSe QDs with Cd:Se ratio 1.0:0.5, 1.5:0.5 and 2.0:0.5 mM (Fig. 3b) have two components (τ₁ and τ₂), which is expected to be originated from the band gap (τ₁) and trap states (τ₂) (Javier et al., 2003). The band gap (τ₁) e-h pair recombine rapidly and have short life time (<10 ns), while the trap state (τ₂) e-h pair recombine slowly having lifetime larger than 30 ns. It was observed that the average lifetime value of the CdSe QDs increases with the increase in the Cd content in the reaction mixture (see Table 1). The QDs containing higher Cd to Se ratio get better capped by starch molecules and hence have better PL and therefore the QDs synthesized with lower Cd:Se ratio 0.5:0.5 and 0.5:1.0 mM have very poor PL.

The QDs were extracted by following a novel colloidal processing method developed in this study. The room temperature colloidal solution of QDs was first freezed at 0 °C and then was allowed to de-freeze to room temperature. The amylose and amylopectin residues present in starch got sedimented after freezing (Fig. 4a) (Spoehr et al., 1936). The colloidal solution of CdSe QDs was also found to get sedimented along with the amylose and amylopectin residues of starch in the bottom of the beaker (Fig. 4b). The sedimented starch along with QDs could be easily extracted out in the powder form (Fig. 4c–e).

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Fig. 4

(a) Colloidal solution of starch; at room temperature (i), after freezing (ii), and again after attaining room temperature (iii), (b) colloidal solution of CdSe QDs capped with starch (i), after freezing (ii) and again after attaining room temperature (iii), Solid CdSe QDs extracted from the reaction mixture synthesizing with (c) 1.5:0.5 mM, (d) 1.0:0.5 mM and (e) 0.5:0.5 mM of Cd:Se precursors.

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3.2 Characterization of CdSe QDs

The XRD pattern of CdSe QDs (Fig. 5a) confirms cubic zinc blende crystal of CdSe (Murray et al., 1993). The crystallite size was calculated by the Scherrer formula and was found to be around 2.0 nm. Raman spectra of the CdSe QDs (Fig. 5b) shows two prominent peaks at 204 and 409 cm⁻¹ which corresponds to 1st and 2nd order longitudinal modes of CdSe nanocrystals (Dzhagan et al., 2008).

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Fig. 5

(a) XRD pattern and (b) Raman spectra of CdSe QDs.

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The TEM image of the particle is shown in Fig. 6a. The image showed that the particles are well dispersed and spherical in size having maximum size around 3 nm. The FFT pattern (inset Fig. 6a) shows the wave vector corresponding to (1 1 1) and (2 0 0) planes with 54⁰ of angle between them. The HRTEM image clearly showed the (1 1 1) and (2 0 0) planes with inter planar distance of 3.5 Å and 2.9 Å respectively, which matches very well with the cubic zinc blende crystal structure of CdSe. The SEM image shows spherical shapes of relatively bigger size, which indicates that these QDs exist in agglomerated form (Fig. 6b).

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Fig. 6

(a) TEM image of CdSe QDs inset 1; particle size distribution, inset 2 FFT pattern and inset 3 HRTEM image showing inter planar distances corresponding to (1 1 1) and (2 0 0) planes, (b). SEM image showing aggregated morphology of CdSe QDs.

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In the FTIR spectra (Fig. 7) the OH stretching peak present in starch (3650–3000 cm⁻¹) is very broad shows the presence of several OH groups, inter and intra molecular hydrogen bonding in starch. The other peaks are at 2900 cm⁻¹ (asymmetric C—H stretching), 1640 cm⁻¹ (O—H bending of water in starch), 1340 cm⁻¹ (angular deformation of C—H bond), 1150 cm⁻¹ (C—O and C—C stretching), 1075 cm⁻¹ (C—O—H bending), 998 cm⁻¹ (skeletal vibration of α 1–4 glyosidic linkage (C—O—C), 926 cm⁻¹ (C—C stretching) and 850 cm⁻¹ (C₂ deformation) (Kizil et al., 2002). The starch capped CdSe samples give distinct peaks which indicate the existence of starch on CdSe surface. The extracted QDs were modified by surface functionalized with thiourea to increase its PL quantum yield and stability. In the FTIR spectra of thiourea functionalized CdSe QDs, the characteristics peaks of thiourea at 3365 cm⁻¹, 3275 cm⁻¹, 3150 cm⁻¹ (symmetric and asymmetric stretching of NH, NH₂), 1600 cm⁻¹ (NH₂ bending), 1400 cm⁻¹ (CN stretching) 1075 cm⁻¹ (NH₂ rocking), 734 cm⁻¹ (CS asymmetric stretching) and 620 cm⁻¹ (CS symmetric stretching) (Stewart, 1957) vanishes in thiourea functionalized CdSe QDs, which confirmed the binding of thiourea on the surface of the QDs. Functionalization with thiourea reduces the spectral broadness of the PL spectrum as well as increases the PL quantum yield (see the supporting information Fig. S5). It is worth to mention here that before functionalization with thiourea the PL spectrum is broad having emission mainly from trap states, however, after functionalization the PL spectrum becomes narrow with very low stokes shift and therefore it could be mainly arising from the band gap recombination of the charge carriers. Further to confirm the functionalization, we recorded the DLS (dynamic light scattering) measurements of starch capped CdSe QDs both before and after functionalization with thiourea. We found that the hydrodynamic diameter of QDs increases after functionalization from 193 nm to 267 nm (see the supporting information Fig. S6). Such an increase in the hydrodynamic diameter clearly indicates the functionalization of QDs (Wenger et al., 2017).

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Fig. 7

FTIR spectra of starch, thiourea, starch CdSe and starch CdSe with thiourea.

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3.3 Effect of temperature and pH on the PL properties of CdSe QDs

The effect of temperature on the PL properties of thiourea functionalized CdSe QDs was studied by recording the temperature dependent PL spectra from 5 °C to 80 °C (Fig. 8a). From these spectra it is clearly evident that QDs have maximum PL at 20 °C. Further on increasing or decreasing temperature the PL yield decreases. Also the PL peak position shifts from 592 nm at room temperature to 600 nm at 80 °C. This red shift can be attributed to minute change in particle size after ligand detachment from the surface of QDs at higher temperature (Valerini et al., 2005). On increasing the temperature, the surface capped molecules get detached from the surface of the QDs and create surface defects which enhance the non-radiative recombination of charge carriers and hence there is a reduction in PL Intensity (Zhao et al., 2012) as well as an increase in broadness (Mohamed et al., 2014). It is a well-known fact that the loss of capping ligands from the surface of QDs creates surface defects.

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Fig. 8

Temperature dependent (a) and pH dependent (b) PL spectra of thiourea functionalized CdSe QDs.

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The PL spectra of thiourea functionalized CdSe QDs were also recorded after changing the pH of the colloidal solution from 3 to 11 (Fig. 8b). It was found that the PL yield decreased on either decreasing or increasing from the neutral pH. At lower pH, the thiourea and starch gets protonated and the QDs become destabilised, hence the PL intensity decreases. At higher pH, the formation of hydrated product on the surface of QDs might cause a reduction in the PL intensity. Furthermore, the metal ions also get precipitated as their corresponding hydroxides at higher pH causing a reduction in the PL intensity/yield (Gore et al., 2011).

3.4 Metal ion sensing studies

3.4.1

3.4.1 Selectivity

Thiourea functionalized CdSe QDs were used in the sensing of various metal ions. 10.0 μM concentration of Hg⁺, Hg²⁺, Co²⁺, Cr³⁺, Cr⁶⁺, Zn²⁺, Pb²⁺, Cd²⁺, Ni²⁺, Mn²⁺, Fe³⁺ or Cu²⁺ were added to the aqueous QDs solution. Out of these metal ions only Cu²⁺, Cr⁶⁺ and Hg²⁺were able to selectively quench the PL intensity of QDs. (Fig. 9). Others metal ions were not able to quench the PL intensity appreciably.

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Fig. 9

The PL quenching effect of different metal ions on the PL of CdSe QDs.

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3.4.2

3.4.2 PL quenching by Cu²⁺, Cr⁶⁺ or Hg²⁺ ions

The effect of Cu²⁺, Cr⁶⁺ or Hg²⁺ ions on the PL quenching of CdSe QDs among various metal ions is established from their PL investigations (see Fig. 10). So in order to understand the PL quenching behaviour of CdSe QDs in the presence of these ions different concentration of Cu²⁺, Cr⁶⁺or Hg²⁺ions were added to the aqueous colloidal solution of CdSe QDs. It was found that the quenching of PL linearly depends upon the concentration of these metal ions (Fig. 10a–c). Therefore, this system can be used as systematic and selective method in sensing of Cu²⁺, Cr⁶⁺ and Hg²⁺ ions as well as the 3+ and 6+ oxidation states of Cr and 1+ and 2+ oxidation state of Hg can be differentiated. The PL quenching data were analysed using Stern-Volmer equation (Lakowicz, 2006) which can be given as follows.

(7)

$$\frac{I_0}{I}={1+K}_{\mathit{sv}}\left[Q\right]=1+k_q{\tau }_0[Q]$$ where $K_{\mathit{sv}}=k_q{\tau }_0$ , I₀ and I are the PL intensity of QDs in the absence and presence of metal ions. K_sv is the Stern-Volmer quenching constant, Q is the concentration of the metal ions, $k_q$ is quenching rate constant and ${\tau }_0$ is the PL lifetime of the QDs in the absence of metal ions. As it is clear from these figures that the plot of I₀/I as a function of metal ion concentration is linear (r² = 0.99) so the Stern-Volmer description of binding of metal ions with the QDs is valid. The quenching rate constant ( $k_q$ ) was determined for all the three metal ions are listed in Table 2.

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Fig. 10

PL quenching of CdSe QDs in presence of different metal ions (a) Cu²⁺, (b) Cr⁶⁺ and (c) Hg²⁺, inset- Stern Volmer plot.

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Table 2 Limit of detection (LOD) and quenching rate constant (k_q) of metal ions.

Metal ion	LOD (μM)	kq (M−1 s−1)
Cu2+	27.6	4.66 × 1014
Cr6+	196.0	8.53 × 1013
Hg2+	279.2	1.32 × 1014

Further, to confirm it, we recorded the time-resolved PL decay profiles and determined the lifetime values of the QDs in the absence as well as in the presence of these metal ions (see the supporting information Fig. S7a–c). It was found that with all these metal ions the PL lifetime decreases with increasing the concentration of metal ions which reaffirm the dynamic nature of the PL quenching of the QDs in the presence of these metal ions.

The limit of detection (LOD) is evaluated by using the formula 3σ/k, (listed in Table 2) where, σ is the standard deviation of the blank signal and k is the slope of the linear calibration plot (validation of analytical procedures: text and methodology Q2(R1), 2015). It is to be mentioned here that LOD for Cu²⁺ ions is lower than that for Cr⁶⁺ and Hg²⁺ ions.

3.4.3

3.4.3 PL quenching mechanism

Several quenching mechanisms have been proposed by various researchers to explain how metal ions quenches the PL of functionalized QDs. Non radiative recombination pathway, inner filter effect, electron transfer process, aggregation induced quenching, metal ion replacement, and ion binding interaction are the possible quenching mechanism to explain the PL quenching phenomena (Du et al., 2017). As we have discussed in the previous section, from the lifetime analysis it is clear that there could be a dynamic PL quenching of the QDs because the PL lifetime values decrease on increasing the metal ion concentration (Lakowicz, 2006). Further to confirm it, we have recorded the absorption spectra of thiourea functionalized CdSe QDs both in absence and the presence of Cu²⁺, Cr⁶⁺ and Hg²⁺ (see the supporting information Fig. S8). The presence of Cu²⁺ and Cr⁶⁺ has no effect on the absorption spectra the QDs but in the presence of Hg²⁺, the excitonic peak position has been slightly red shifted. This slight red shift in the absorption spectra may be due to nominal increase in the size of QDs in the presence of Hg²⁺ which causes PL quenching (Du et al., 2017). The quenching rate constant, k_q (Table 2) for all the three metal ions are almost 10³–10⁴ times higher than the diffusion-controlled quenching rate constant. This shows that the quenching could also be static in nature even though there is no significant change in the absorption spectra of the QDs in the presence of these metal ions (Ding et al., 2015). So from the decrease in the PL lifetime values of QDs with increase in metal ion concentration and the high quenching rate constant obtained in this case, we suggest that the PL quenching could be assigned as both static as well as dynamic in nature. In previous literature, it is reported that the metal ions can interact with QDs through the capping layer and can quench their PL (Wu et al., 2008; Yu et al., 2012). Thiourea has two binding sights through which it can bind with the QDs through the (i) S atom and (ii) two N atoms. It is already confirmed by the FTIR spectra that both N and S atoms of thiourea are involved in the functionalization of CdSe QDs, but the S atom will be predominantly bound to Cd atom of the QDs (according to HSAB principle: soft acid (Cd) and soft base (S)), leaving the N uncoordinated.

Scheme 1 depicts the possible bindings of thiourea with QDs and metal ions. Being a borderline acid Cu²⁺ can bind with both S and N atom while Hg²⁺ and Cr⁶⁺ can only bind with S and N atom, respectively. Thus, Cu²⁺ has a strong affinity towards thiourea and hence a lower limit of detection and higher quenching rate constant as compare to Cr⁶⁺ and Hg²⁺. It is seen that Cu²⁺, Cr⁶⁺ and Hg²⁺ ions predominantly form complexes with thiourea on the CdSe QDs surface, which results in the imperfection of their surface and facilitate the non-radiative e/h recombination. The higher quenching rate constant also favours the association of the metal ions directly to the surface of QDs and photo-induced e transfer from the conduction band of QD to the metal ions.

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Scheme 1

Possible bindings metal ions with thiourea attached to the surface of CdSe QDs.

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3.4.4

3.4.4 Interference effect of Pb²⁺, Cd²⁺and Fe³⁺ ions

In order to adopt the QDs as a probe for these metal ions it is very essential to study the interference effect of certain other ions. This interference study is focused on the cations which are naturally found along with these metal ions or have adverse effect on human being or environment. Pb²⁺ and Cd²⁺are one of the most hazardous heavy metals along with Hg²⁺. Fe³⁺ is the constituent metal ion which is found along with copper (chalcopyrite; CuFeS₂) and chromium (chromite; FeCrO₄). So interference of Pb²⁺, Cd²⁺ and Fe³⁺were studied by taking the equal or ten times higher concentration of these interfering ions. From Fig. 11 it is clear that the Pb²⁺ is showing very little interference with Cu²⁺ and Hg²⁺ while it is showing adequate interference with Cr⁶⁺. On the other hand, Cd²⁺ is showing very little interference with Cu²⁺ and Cr⁶⁺ but showing good interference with Hg²⁺. Fe³⁺ is showing good interference with Cu²⁺ and Hg²⁺.

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Fig. 11

Histograms showing the change in PL of CdSe QDs in the presence of Pb²⁺, Cd²⁺ or Fe³⁺ with Cu²⁺, Cr⁶⁺ and Hg²⁺(equal concentration and ten times higher concentration of interfering metal ions).

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