Characterization and synthesis of Ni–doped CsPbBr
3
NCs@SiO
2
Ni–doped CsPbBr3 NCs@SiO2 nanocomposite with high efficiency was prepared using a ligand reprecipitation route. The surfactant ligands as well as reaction temperature play crucial role in determining the shape, size, and surface properties of colloidal perovskite nanocrystals2. The acid–base organic ligands system was provided through oleic acid and oleylamine with long carbon chain.Pristine CsPbBr3 NCs have poor colloidal stability and relatively low quantum yield. In addition to surface passivation, doping metal cations through cation exchange followed by anion exchange to the crystal lattice of perovskites can induce new optical properties and improve their stability4. According to Fig. 1a, the pristine CsPbBr3 NCs demonstrate green emission at the wavelength of 525 nm, with the PL quantum yield (PLQY) of approximately 25%. However, with the introduction of Ni2+ into the structure of the CsPbBr3, the Ni–doped CsPbBr3 NCs with sharp green emission at 535 nm along with greater QY (64%) is obtained. As indicated in the Fig. 1b and Table S2 (ESM), the QY of the PL gradually increases with the rise of Ni to Pb mole ratio. On the other hand, at higher nickel concentrations, the emission blue shifts from 535 to 516 nm probably due to the quantum confinement effect. The distinct size difference between Pb2+ (radius ~ 1.19 Å) and Ni2+ (radius ~ 0.69 Å) probably creates specific planar defects in the doped perovskites structure. Excitons are confined within domains between these defects. Since the size of domains between defects is reduced by increasing dopant concentration, a slight blue shift in the PL peak is observed 26. As illustrated in Fig. 1a, addition of Ni2+ to the perovskite structure significantly improves the optical properties of these NCs; when the Ni: Pb feeding ratio is increased to 2.0, the PLQY increases remarkably up to 64% (as measured by using fluorescein (QY = 92%) as a reference). Figure 1c exhibits the absorption spectra of undoped CsPbBr3 NCs and Ni–doped CsPbBr3 NCs@SiO2 with varying amounts of Ni2+ (with a molar ratio of Ni:Pb = 0.5:1, 1:1, 2:1, 3:1, and 4:1). By addition of Ni2+ ions to the crystal structure of perovskite, the band gap energy calculated according to Tauc plot, is almost constant for Ni:Pb ratios of up to 3:1 (around 2.36 eV). However, at Ni:Pb ratio of 4:1 an increase in band gap to 2.47 eV is observed.Fig. 1(a) PL spectra of undoped and Ni–doped CsPbBr3 NCs and Ni–doped CsPbBr3 NCs embedded in SiO2. (b) PL spectra of Ni–doped CsPbBr3 NCs@SiO2 at various Ni: Pb mole ratio. (c) UV–Vis spectra of Ni–doped CsPbBr3 NCs@SiO2 at various Ni: Pb mole ratio.Transmission electron microscopy (TEM) of Ni–doped CsPbBr3 NCs embedded in SiO2 mesoporous scaffolds is presented in Fig. 2. The average diameter of the nanoparticles was determined to be 36 ± 4 nm. Closer look at these particles shows small NCs formed in the pores of SiO2. The TEM images of pristine CsPbBr3 NCs as well as Ni–doped CsPbBr3 NCs prepared in order to compare their behavior, are given in Fig. S1 (ESM). Their average diameters are 17 ± 5 nm and 21 ± 5 nm, respectively. In situ growth of NCs in the SiO2 pores leads to size control and reduction of particle aggregation. According to TEM results (inset of Fig. 2), the sizes of Ni–doped CsPbBr3 NCs formed in the pores are much smaller than free NCs which is due to the small size of SiO2 pores. This is consistent with previous reports27 The crystal structures of CsPbBr3 NCs, Ni–doped CsPbBr3 NCs and Ni–doped CsPbBr3 NCs@SiO2 were determined using X–ray diffraction (XRD). According to the XRD patterns (Fig. 3), the main peaks of (100), (110), (200), (210), (211), and (220) were observed at a 2θ of about 15.4°, 21.8°, 30.8°, 34.5, 37.9°, and 43.9°, respectively. The results showed that all samples had the same crystal structure as the cubic phase CsPbBr3 (COD ID: 1,533,062), and there was no overall structural change after doping (Fig.S2). Based on these results, it seems that Ni2+ ions are efficiently replacing Pb2+ ions in the CsPbBr3 lattice9,28. The elemental composition, mapping (Fig. S3), and comparative FTIR spectra (Fig.S4) of CsPbBr3 NCs, Ni–doped CsPbBr3 NCs and Ni–CsPbBr3 NCs@SiO2 are reported in detail in ESM.Fig. 2TEM image of Ni–doped CsPbBr3 NCs@SiO2 (inset is the magnified image of a single particle showing NCs in pores of SiO2).Fig. 3XRD patterns of undoped CsPbBr3 NCs, mesoporous SiO2, Ni–doped CsPbBr3 NCs, and Ni–doped CsPbBr3 NCs@SiO2.We comparatively investigated the stability of synthesized NCs. As shown in Fig. S5a, in the case of undoped CsPbBr3 NCs, a non-luminescent orange precipitate formed after 5 days which the implies its instability and phase change. Unwrapped Ni–doped NCs stayed stable for about two weeks. As can be seen from Fig. S5b, after 16 days a large shift in PL wavelength and a decrease in its intensity observed. On the other hand, Ni–doped CsPbBr3 NCs@SiO2 demonstrated exceptional durability for almost one month in isopropyl alcohol, with a loss of less than 14% of initial PL intensity after 30 days (Fig. S5c). Finally, as shown in Fig.S6, the PL intensity of Ni–doped CsPbBr3 NCs@SiO2 dispersed in isopropyl alcohol remains almost unchanged after injection in water for 60 min. It is clear that the insertion into mesoporous SiO2 provides good stability for Ni–doped CsPbBr3 NCs in water.
Direct chemiluminescence of Ni–doped CsPbBr
3
NCs@SiO
2
Firstly, we examined the CL reaction of Ni–doped CsPbBr3 NCs@SiO2 with various concentrations of commonly used oxidants (including Ce(IV), KMnO4, H2O2, KIO4 and K3Fe(CN)6). According to the achieved results, all oxidants except KIO4, can directly elicit a CL from Ni–doped CsPbBr3 NCs@SiO2. As illustrated in Fig. 4, Ce(IV) produces the most intense CL signal, so Ce(IV) was picked as the oxidant for the CL reaction. The exact reason why Ce(IV) gives highest CL intensity is not known for us, but better matching of energy produced during the oxidation reaction with Ce(IV) to the excitation energy required to form the excited state of Ni–doped CsPbBr3 NCs@SiO2 may explain this phenomenon. It should be mentioned that high concentrations of KMnO4, leads to a remarkable decrease in the CL intensity which can be attributed to its self-absorption effect.Fig. 4Effect of different concentrations of oxidants on the CL intensity of Ni–doped CsPbBr3 NCs@SiO2 (200 µL). All oxidants were in basic medium (NaOH, 0.1 M) except KMnO4 that was in acidic medium (H2SO4, 0.02 M), and Ce(IV) which prepared in sulfuric acid medium (0.2 M).As shown in the CL kinetic profile (Fig. 5), the maximum intensity of direct CL of Ni–doped CsPbBr3 NCs@SiO2 induced by Ce(IV) is reached within 1 s after injection of the oxidant. The normalized kinetic profiles given in Fig. S7 (ESM) better show this point. For comparison, the CL profile of undoped and Ni–doped NCs in reaction with Ce(IV) is also given in Fig. 5 (each in their optimal conditions). As can be seen, the signal increases in order of undoped CsPbBr3 NCs < < Ni–doped CsPbBr3 NCs ~ Ni–doped CsPbBr3 NCs@SiO2. This is consistent with PLQYs of NCs (25% for undoped CsPbBr3 NCs compared to 64% and 58% for Ni–doped CsPbBr3 NCs and Ni–doped CsPbBr3 NCs@SiO2, respectively). Slightly higher CL of nanocomposite compared to Ni–doped CsPbBr3 is probably due to its porous nature, which leads to a more effective reaction. All kinetic profiles were obtained by automatic monitoring of CL signal over time after the injection of oxidant. This was carried out automatically by the luminometer instrument. It is important to note that each NC was studied under its optimal conditions. The faster signal decay rate in both nickel-doped NC samples may be attributed to their higher catalytic effect compared to undoped one.Fig. 5CL kinetic profiles for Ce(IV) (3.5 mM)–CsPbBr3 NCs (250 µL), Ce(IV) (2 mM)–Ni–doped CsPbBr3 NCs (200 µL) and Ce(IV) (1.5 mM)–Ni–doped CsPbBr3 NCs@SiO2 (200 µL).The role of primary ingredients of the synthesis process of nanocomposite (DMF, OAm, OA and 2–propanol) on the CL signal was investigated. As shown in Table S3, the precursor mixture did not have a significant influence on the CL. Therefore, the CL response is solely due to the Ni–doped CsPbBr3 NCs@SiO2. Based on the kinetic profiles, the direct oxidation of Ni–doped CsPbBr3 NCs@SiO2 by Ce(IV) occurred very quickly, so CL signal peaked within 1 s after the oxidant was injected into the solution. The response then began to decline and eventually disappeared after approximately 15 s. In contrast, the CL signal of undoped CsPbBr3 NCs with Ce(IV) reached its maximum within 2 s and diminished after about 5 s.Possible reaction mechanism for CLIn order to determine the CL emitter as well as inspect the reaction mechanism, the CL spectrum of Ni–doped CsPbBr3 NCs@SiO2–Ce (IV) was obtained using a spectrofluorometer. For this purpose, the excitation source was turned off during the recording. According to Fig. 6 a CL peak emerges at around 545 nm which was comparable with the PL spectrum of Ni–doped CsPbBr3 NCs@SiO2. This indicates that the CL originates from the excited–state Ni–doped CsPbBr3 NCs@SiO2.Fig. 6CL spectrum of reaction (a) in the absence and (b) in the presence of Ni–doped CsPbBr3 NCs@ SiO2. Inset (c) is the PL spectrum of Ni–doped CsPbBr3 NCs@SiO2.To verify the reaction between Ni–doped CsPbBr3 NCs@SiO2 nanocomposite and Ce(IV), the PL spectrum of nanocomposite was recorded after mixing with this oxidant. As can be seen from Fig. S8 (ESM), the fluorescence peak of Ni–doped CsPbBr3 NCs@SiO2 at 535 nm decreases distinctly upon addition of Ce(IV) to the solution. We concluded that the reaction mechanism was a direct oxidation process. During the process, both electron and hole was created due to the chemical reaction. Some electrons could be thermally excited to the conduction band of Ni–doped CsPbBr3 NCs@SiO2, while Ce(IV) as a strong oxidant, could inject holes into its valence band. Due to the recombination of the electron–hole, energy is released and light emission occurs. A schematic diagram illustrating electron–hole recombination is depicted in Fig. S9 (ESM).By removing dissolved oxygen from the solution using nitrogen bubbling, the CL signal decreased by a small amount (less than 5%), which shows that dissolved oxygen does not have significant effect on the CL emission.Analytical application of CL system to L–Cys assayWhen L–Cys is added to the system, there is a noticeable increase in the intensity of the CL signal. The high affinity of thiol group in L-Cys to perovskite NCs leads to a significant suppression of surface defects in the Ni–doped CsPbBr3 NCs@SiO2 nanocomposite structure29,30. According to Fig. S10, the PL intensity of Ni–doped CsPbBr3 NCs@SiO2 nanocomposite intensified with the addition of L–Cys, confirming this assumption. Thiol groups may be coordinated to the surface of the NCs at the Pb–rich defect sites31. The presence of Pb–S bond is confirmed through a distinct peak at around 804 cm-1 in the FTIR spectrum of perovskite NC in the presence of L-cysteine (Fig. S11)31. Anyway, the strong interaction of L-Cys with perovskite NCs can lead to surface passivation, reduction of non-radiative recombination centers, or alteration of energy transfer processes within NCs, all of which can contribute to the increase in CL intensity.In order to attain the highest sensitivity in the determination of the L–Cys, the optimum quantity of SiO2 in the synthesis process was investigated with various molar ratios of Ni to Pb. According to experimental results (Table S4), with 0.01 g of SiO2 and Ni to Pb ratio of 2:1, the probe has the highest sensitivity toward L–Cys. Other experimental parameters were also optimized. According to the results (Fig. S12–S13), 1.5 mM of Ce(IV) and 200 µl of Ni–doped CsPbBr3 NCs@SiO2 were picked as optimum values.As exhibited pictorially in Fig. 7 under optimal experimental conditions, by increasing the amount of L–Cys from 20 to 300 nM the CL intensity of Ce (IV)–Ni–doped CsPbBr3 NCs@SiO2 was linearly intensified. The related linear equation was I/I0 = 0.0037CL-Cys + 0.961 with R2 = 0.9991, where I/I0 is defined as the CL signal in the absence (I0) and in the presence (I) of L–Cys. In this way, the detection limit of the probe was calculated to be 12.8 nM based on 3sy/x/b (where sy/x is standard error of the regression, and b is the slope of the calibration curve)32. In order to study the repeatability of the established probe, the intra–day and inter–day precision as relative standard deviation (RSD) for seven replicate determinations of 75 nM L–Cys were calculated, which were 2.0% and 3.1% respectively. We also investigated the enhancing impact of L–Cys on the direct CL system of pristine CsPbBr3 QDs and Ni–CsPbBr3 NCs with Ce(IV) as oxidant. Table S5 (ESM) indicates that the use of these NCs creates calibration plots with more restricted linear ranges and higher detection limits and poor sensitivities than Ni–doped CsPbBr3 NCs@SiO2. It is interesting that the performance of nanocomposite is not only better than pristine perovskite but also than Ni–doped perovskite. This is probably due to the porous nature of SiO2 which leads to the accumulation of L–Cys molecules and their more effective interaction with Ni–doped CsPbBr3 NCs. Moreover, the poor repeatability of the signals obtained from Ni–doped CsPbBr3 NCs, indicates that its stability in water is low and cannot be relied upon. Anyway, the results suggest that Ni–CsPbBr3 NCs@SiO2 not only have a superior enhancing effect on the CL reaction but also have a higher affinity to L–Cys. These results specify that the performance of encapsulated perovskite NCs has been expressively improved compared with that of bare perovskite NCs. Table 1 compares the performance of our developed method for L–Cys with that of other CL methods. Our assay has equal or higher analytical performance compared to most reported CL–based methods.Fig. 7Time profiles of Ce(IV)–Ni–doped CsPbBr3 NCs@SiO2 CL system with various concentrations of L–Cys and the related calibration plot (inset).Table 1 Comparing the proposed method with some previously reported CL methods for the determination of L–Cys.Selectivity of the designed CL probe was evaluated against some similar biological molecules and common inorganic ions that may found in serum samples. The achieved results are outlined in Table 2. As can be seen, the interference effect of most species in L–Cys determination was negligible. Homocysteine, a homolog of the cysteine with additional methylene bridge (–CH2), have no significant influence on the determination of L–Cys. It should be mentioned that, the concentration of L–Cys in human serum is naturally 240–360 µM, while that of homocysteine is normally below 12–15 μM33. So our method has an excellent selectivity for L–Cys, which may be attributed to the particular interaction of L–Cys with Ni–doped CsPbBr3 NCs@SiO2 which was discussed before.Table 2 Interferences of several common ions and biological species in determining L–Cys (150 nM).It should be mentioned that current probe is quite stable in polar environments and produces reproducible results. However, since it is used in the liquid phase reactions, each test sample requires a separate probe.Analysis of real sampleNi–doped CsPbBr3 NCs@SiO2–Ce(IV) CL probe was exploited to determine L–Cys in various serum samples and the results are reported in Table 3. First, the L–Cys present in the samples was measured, then to confirm the accuracy of the method, the serum samples were spiked with specific concentration of L–Cys. The recoveries as well as Student t-tests are demonstrated the appropriate correlation of the achieved values. According to the satisfying recoveries from 92.5 to 108.0% with RSD of 1.3 to 7.2%, the fitness of the established CL probe for selective and sensitive assay of L–Cys in actual samples was approved. The accuracy of proposed method was further confirmed via HPLC method. The results are given in Table 3 and corresponding chromatograms are shown in Fig. S14.Table 3 Determination of L–Cys in serum samples.
An efficient chemiluminescent probe based on Ni-doped CsPbBr3 perovskite nanocrystals embedded in mesoporous SiO2 for sensitive assay of L–cysteine
