Highly stable and water dispersible polymer-coated CsPbBr 3 nanocrystals for Cu-ion detection in water †

Recently, CsPbX 3 (X = I, Br, Cl) nanocrystals (NCs) have shown huge potential in the fields of various optoelectronic applications. However, the CsPbX 3 NCs degrade very rapidly in the presence of water and heat, because of dynamic oleic acid and oleylamine capping ligands. The silica-coated CsPbBr 3 NCs are comparatively stable but the synthesis is slightly complicated and needs several hours to complete the reaction process. These silica-coated NCs also tend to agglomerate among themselves, which is unfavorable for display technologies and bioimaging applications. In these regards, we introduce novel polymers [polyvinylpyrrolidone and n -isopropyl acrylamide] along with oleic acid and oleylamine that are encapsulated around the CsPbBr 3 NCs during the synthesis procedure at room temperature. Such NCs are highly luminescent in the green spectral region with a maximum photoluminescence quantum yield of up to 93%, have narrower emission spectra, and can easily disperse in water. The size distribution of the polymer-coated NCs becomes more uniform and well-dispersed. The water dispersity and stability of these polymer-coated NCs significantly improved in comparison to the conventional silica-coated NCs. Such water-stable NCs were tested as a luminescent probe for Cu 2+ -ion detection in water that shows a detection limit of 18.6 m M. These outcomes are very encouraging for futuristic display technologies, bioimaging, and sensing applications.


Introduction
9][20] In recent times, many different NCs have evolved as promising materials for disease diagnostics and therapeutics. 21,22The definite size, shape, and/or surface chemistry of the NCs exhibit their specific functionalities that can be tailored for different requirements.The size of the NCs affects the uptake efficiency through the different cell lines and kinetics to reach the targeted organ or tissue cells. 23,24The optimal NCs size should be below 50 nm for active uptake into live cells.
31]35 Most of these experiments were performed in nonpolar solvents to avoid the degradation of perovskite NCs due to their poor structural stability. 36,37These non-polar solvents are immiscible in aqueous media, which restricts metal ion detection in the biological systems or natural resources.So, highly water stable and water dispersible fluorescent probes are required such that the detection efficiency of metal ions can be improved.The perovskite NCs are usually encapsulated with oleic acid (OAc) and oleylamine (OAm) ligands, which are very dynamic and unstable.So, the NCs are very prone to degradation in the presence of heat, water, or intense UV light.It's a basic need to improve the perovskite structural stability for implementing them in real-world activities.Shelling of perovskite NCs with different stable materials is an effective way to further improve the NCs' stability and luminous intensity.These shelling materials protect the core from external harsh environments and maintain high emission intensity and stability for a longer time period.Bhaumik et al. synthesized methylammonium lead bromide (MAPbBr 3 ) core and layered octylammonium lead bromide [(OA) 2 PbBr 4 ] shell NCs by a ligand-assisted reprecipitation (LARP) synthetic method, demonstrating better structural stability and luminescence intensity that persisted for several months. 381][52] The polymers are usually flexible and increase the hydrophobicity of NCs, which improves the water stability.Polyvinyl-pyrrolidone (PVP) polymer encapsulated CsPbBr 3 NCs embedded in a polystyrene (PS) matrix exhibited better water resistance and were demonstrated as luminescent probes in live cells. 53PVP-coated CsPbBr 3 NCs surrounded by a polymethyl methacrylate (PMMA) matrix were also explored for live-cell imaging. 54][57] Silica-coated perovskite NCs are mostly explored due to their better stability in water. 48,58This synthesis procedure required a longer reaction time for the completion of the hydrolysis and condensation processes of silane groups for the growth of a thick silica shell around the NCs.The silica-coated NCs also tend to agglomerate among themselves and become bigger particles for reduction of surface energy, resulting in non-uniform particle size distributions. 42,59These agglomerations among NCs also lead to an increase in NCs reabsorption and scattering, causing a decrease in the NCs luminous intensity and device efficiency.
Here in this work, we tried to find an easy and robust synthesis protocol that can be suitable for industrial processing.We report the synthesis of CsPbBr 3 NCs via a one-pot LARP synthetic approach under normal atmospheric conditions (relative humidity level above 80%).Here, we encapsulated the CsPbBr 3 NCs with different stable materials, such as SiO 2 , PVP, or mixed PVP-NIPAM polymers, and then investigated their structural and emission properties.In the case of silica-coated NCs, we selected (3-aminopropyl)trimethoxysilane (APTMS) as a silica source because APTMS binds the CsPbBr 3 NCs surface with their amino groups while the alkoxysilane groups hydrolyze to form silanol groups in the presence of a trace amount of water in toluene solvent during the synthesis process. 48,60Furthermore, silanol groups condensate to form silica coatings around the NCs.In the case of the polymer coating, PVP and NIPAM polymers were used as shelling materials that attach to the NC surface and help to control the NCs growth.PVP gets adsorbed to the surface of the NCs, and further, NIPAM and PVP intertwine among themselves to form a protective shell that protects the NCs core from the harsh atmosphere.OAc and OAm ligands are also added during the NCs synthesis process such that OAc ligands prevent agglomeration among the perovskite NCs, while the amino groups of the OAm ligands attach to the NCs surface and improve NCs solution stability. 61Finally, we used the most stable polymer-coated NCs as a luminescence agent to investigate their Cu 2+ -ion detection ability in water.Since the degradation of perovskite NCs becomes slower in water for effective polymer coating, this permits the detection of Cu 2+ -ions directly in water with high sensitivity.This work paves a way to detect Cu 2+ -ions in biological systems or natural resources.

Synthesis of SiO 2 -coated CsPbBr 3 NCs
Silica-coated CsPbBr 3 NCs were synthesized via a modified ligand assisted re-precipitation (LARP) method. 48First, three separate precursor solutions were prepared by dissolving 0.02 mmol (4.2 mg) CsBr in 250 mL of DMF, 0.02 mmol (4.2 mg) OABr in 250 mL of DMF, and 0.02 mmol (7.34 mg) PbBr 2 in 250 mL of DMF and mixed properly via stirring to complete dissolution.Next 200 mL of CsBr precursor and 50 mL of OABr precursor (CsBr : OABr = 8 : 2) were injected into the PbBr 2 precursor solution and mixed properly.Finally, 10 mL of APTMS was injected into the PbBr 2 precursor to form the final precursor.250 mL of the final precursor was quickly injected dropwise in a round bottom flask containing 5 mL of toluene under vigorous stirring conditions.The stirring was continued for 4 hours to complete the hydrolysis process of APTMS.The NC solution was then centrifuged at 3000 rpm for 15 min.The supernatant was discarded and the precipitate was dispersed in 1 mL of ethanol for further characterization.The NCs are renamed CPB-SiO 2 for better understanding.
Synthesis of PVP and mixed PVP-NIPAM coated CsPbBr 3 NCs PVP-coated CsPbBr 3 NCs were synthesized via a modified LARP method. 48At first, three separate precursor solutions were prepared by dissolving 0.02 mmol CsBr in 250 mL DMF, 0.02 mmol OABr in 250 mL DMF, and 0.02 mmol PbBr 2 in 250 mL DMF under vigorous stirring conditions.Next, 200 mL of CsBr precursor and 50 mL of OABr precursor (CsBr : OABr = 8 : 2) were injected into the PbBr 2 precursor solution and mixed properly.Finally, 50 mL of OAc and 25 mL of OAm were added to the mixture solution to form the final precursor.250 mL of the final precursor was injected dropwise into a round bottom flask containing 5 mL toluene and 4 mg PVP under vigorous stirring.The reaction was continued for 30 min and then the reaction was stopped.The resulting CsPbBr 3 NC solution (renamed PbN-1) was transferred to a centrifuge tube and then 2 mL ACN was added.The mixture was centrifuged at 6000 rpm of speed for 15 min and the precipitate was dispersed in 1 mL toluene or water for further characterization.The mixed PVP-NIPAM coated CsPbBr 3 NCs were synthesized following the same method with slight modification in which the amount of PVP (B4 mg) was kept fixed while variable amounts of NIPAM (5 mg: PbN-2, 10 mg: PbN-3, 15 mg: PbN-4, and 20 mg: PbN-5) were added.

Results and discussion
Schematic diagrams of the encapsulation process of CsPbBr 3 NCs with silica, and coating with PVP and NIPAM polymers during the synthesis process are shown in Fig. S1a and b  The X-ray diffraction (XRD) pattern of CPB@SiO 2 and different PbN NCs in thin-film form are demonstrated in Fig. 1.The diffraction pattern for the CPB@SiO 2 NCs thin-film reveals mixed perovskite phases, i.e., 3D CsPbBr 3 (monoclinic phase) and 0D Cs 4 PbBr 6 (rhombohedral phase) crystal structures.Similar mixed-phase perovskite NCs were already reported when the silica-coated CsPbBr 3 NCs were synthesized via the LARP synthesis method. 10,62Significant XRD diffraction peaks are observed at 2y values of 15.451, 21.761, 30.561, 34.71, 37.941, and 44.381 that correspond to the (100), ( 110), (002), ( 201), (211), and (202) lattice planes of the 3D monoclinic CsPbBr 3 crystal structure (PDF #00-18-0364). 10,62 113), (300), ( 024), ( 131), ( 214), ( 223), ( 134), (324), and (600) lattice planes of the rhombohedral Cs 4 PbBr 6 crystal structure (PDF #01-73-2478). 50,63The contribution of rhombohedral Cs 4 PbBr 6 phases is quite high as compared to monoclinic CsPbBr  213), (314), and (207) lattice planes of the tetragonal CsPb 2 Br 5 crystal structure (PDF#25-0211). 45,64With an increase in NIPAM amount (i.e., PbN-2, PbN-3, PbN-4, and PbN-5 NCs) in the PbN-1 NCs, there is a minimal change in the monoclinic CsPbBr 3 crystal phases.In contrast, an abrupt reduction in rhombohedral Cs 4 PbBr 6 and tetragonal CsPb 2 Br 5 crystal phases is noticed.The XRD diffraction peaks related to (OA) 2 PbBr 4 perovskite phases can't be detected due to the formation of a very thin shell layer and below the XRD instrument detection limit. 38,48he shape and size of the NCs are investigated using transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) imaging.The TEM and HRTEM images of CPB@SiO 2 and PbN NCs are shown in   2a) appeared from the adsorption of the silica coating that is compact and has a shell thickness of a few nm.The silica shells diffuse among other silica-coated NCs and transform into bigger irregular shapes.The average particle size distribution of the CPB@SiO 2 NCs is 32 AE 7 nm, which is quite big in size.These NCs exhibit inter-planar spacing of 0.42 nm that corresponds to the (110) lattice planes of monoclinic CsPbBr 3 crystal phases. 65In comparison, the polymer-coated PbN NCs (see Fig. 2b-f) reveal a monodisperse and uniform particle size distribution without any noticeable inter-particle agglomeration.These PbN NCs are cubic in shape and highly crystalline in nature.A similar (110) lattice plane appeared for the PbN NCs from the monoclinic CsPbBr 3 crystal structures.The mean particle sizes of PbN-1, PbN-2, PbN-3, PbN-4, and PbN-5 NCs are around 13.47 AE 3, 13.2 AE 3, 13.1 AE 3, 10.7 AE 2, and 10.2 AE 2 nm, respectively (see Fig. S2 in the ESI †).It can be seen that the particle size becomes smaller and the particle size distribution is more uniform with an increase in the amount of NIPAM polymer.It is beneficial for strong quantum confinement of charge carriers inside the NCs and hence enhances the optical properties.
The UV-vis absorption spectra of all the NCs in the solution phase are shown in Fig. 3a.The absorption spectrum of CPB@SiO 2 reveals a band edge peak at 507 nm originated due to 3D CsPbBr 3 perovskites and a minor peak at 320 nm that appeared from 0D Cs 4 PbBr 6 perovskites. 11The PbN NCs demonstrate nearly similar absorption spectra while more intense, sharper, and well-defined absorption band edges are observed.The scattering effect of CPB@SiO 2 NCs is much higher as compared to PbN NCs because of the bigger particle sizes of CPB@SiO 2 NCs. 66,67The band edge absorption peaks of PbN NCs are slightly redshifted with increasing the NIPAM polymer.The PL emission spectra of CPB@SiO 2 NCs (dispersed in ethanol) and PbN NCs (dispersed in toluene) are shown in Fig. 3b.The corresponding photographic images of all NCs under a UV lamp are shown in the inset of Fig. 3b.The CPB@SiO 2 NCs emit a cyan-green color emission with a PL peak position at 494 nm.These NCs showed emission PLQY up to 63% with an FWHM of 31 nm.The low emission intensity of the CPB@SiO 2 NCs is resulted due to the presence of unwanted Cs 4 PbBr 6 phases that don't contribute to the radiative recombination.The polymer-coated PbN NCs are more luminescent compared to silica-coated NCs due to the reduction of the Cs 4 PbBr 6 phases.The PbN NCs also reveal narrower emission spectra and better emission intensity, denoting efficient radiative recombination and higher color purity.The emission peak for PbN-1 NCs is obtained at 513 nm with a very narrow FWHM of 21 nm.Initially, with the addition of NIPAM polymer up to a certain limit, the emission intensity of the PbN NCs was increased.The further increment of NIPAM polymer resulted in a reduction of the NCs PL intensity.The binding constant of the NIPAM was estimated from the change in the emission intensity of the PbN NCs with an increase in NIPAM concentration. 68The binding constant was calculated from the intercept of the plot and it comes close to 8787 M À1 (see Fig. S4, ESI †).Among all these PbN NCs, the PbN-3 NCs demonstrate the highest emission intensity with a maximum PLQY up to 93%.The CIE color coordinates of the CPB@SiO 2 and PbN-3 NCs are calculated as (0.071, 0.395) and (0.079, 0.723), respectively (see Fig. S3 in the ESI †).The subsequent information regarding the PL peak position, FWHM, and PLQYs of all NCs is tabulated in Table 1.
We performed Fourier transform infrared spectroscopy (FTIR) analysis for all NCs, and the subsequent plots are represented in Fig. 3c.The spectrum for CPB@SiO 2 NCs reveals strong absorption peaks located at 1107 and 754 cm À1 due to antisymmetric stretching of Si-O-Si bonds and symmetrical stretching of Si-O bonds, respectively. 59,60The additional peaks at 2927 and 2874 cm À1 appeared due to asymmetric and symmetric stretching of C-H bonds, respectively.The enrichment of  the Si-O-Si and Si-O vibration peaks confirms the silica coating around the NCs after the hydrolysis process.][55][56] These vibration peaks confirm the presence of a polymer coating around the PbN NCs.There is also an absorption peak at 3100 cm À1 corresponding to -OH stretching vibrations from the presence of OAc.Additionally, the absorption peak intensities at 1650-1660 and 3500 cm À1 became intense with an increase in NIPAM polymer (PbN-4 NCs) compared to without NIPAM (PbN-1 NCs), revealing stronger CQO bonding interactions and N-H bonding vibrations, respectively.There was also a decrease in peak intensity at 3100 cm À1 with an increase in NIPAM polymer in the PbN-1 NCs, revealing the weakening of the -OH bonds.
The PbN NCs can easily be dispersed in water and maintained their luminescence properties for several days.The photographic image of the water dispersed PbN NCs under a UV lamp is shown in Fig. S5 (in the ESI †).We examined the water stability of the CPB@SiO 2 and PbN-4 NCs in thin-film form by dipping them into a glass beaker containing DI water.The photographic images of the thin-films under a UV lamp are shown in Fig. 3d.The silica-coated NCs tend to agglomerate among themselves on the glass substrate and result in nonuniform film morphology.After dipping the CPB@SiO 2 NCs thin-film into DI water, the luminescence of the NCs quenched instantly.In comparison, the coverage of the PbN-4 NCs thinfilm is relatively better, and a smooth surface was formed due to lower surface energy after polymer coating, which is a prerequisite for the fabrication of efficient thin-film based LEDs. 69,70The emission intensity of the PbN-4 NCs thin-film was retained for a longer time period, demonstrating ultra-high stability of the PbN NCs films in water.These results conclude that the coating of PVP and NIPAM polymers around the NCs significantly improves the NCs emission intensity and crystal structural stability.
Furthermore, we performed a water stability test of all NCs in the solution phase.At first, we prepared a 5 mg mL À1 concentrated CPB@SiO 2 NCs solution dispersed in ethanol, and the same concentration PbN NCs solution dispersed in water (since toluene and water are immiscible).We started with 100 mL of each NCs solution in separate glass vials followed by the addition of 50 mL of DI water in sequence.We measured the amount of DI water required to completely degrade the perovskite structures, i.e., when the luminescence from the NCs was mostly quenched (PL intensity nearly zero).The PL spectra of the CPB@SiO 2 , PbN-1, and PbN-4 NCs for the water-stability test are shown in Fig. 4a-c and the remaining NCs are represented in Fig. S6 (in ESI †).The CPB@SiO 2 NCs degrade much faster, in which the emission color started shifting from green to blue color with addition of just 50 mL DI water (PL quenched by 73%) and the total emission was quenched in the presence of 250 mL DI water (around 96% of PL quenching).The blue emission spectrum is originated from transformation of 3D perovskite structures to comparatively stable lower dimensional perovskite structures. 71,72The water stability of the polymer coated PbN NCs is significantly improved with increasing the NIPAM polymer and hardly any emission shift is noticed.Fig. 4d shows the change in PL intensity for all NCs with the addition of DI water.The PL retention percentages for CPB@SiO 2 , PbN-1, PbN-2, PbN-3, PbN-4, and PbN-5 NCs are 4%, 23%, 31%, 50%, 57%, and 53%, respectively, with addition of 250 mL DI water in the corresponding NCs solutions.The PbN-4 NCs exhibit the best water stability.The emission intensity of the PbN-4 NCs remains 21% of the original PL intensity even after the addition of 1000 mL DI water.The photographic images of all the NCs solutions after the addition of DI water are shown in Fig. S7 (in the ESI †).
We also performed a water stability test for all NCs in powder form as a function of time.Here, 100 mL of DI water was added to the same amount of NC powders and kept under a UV lamp.Then we recorded the changes in emission spectra at regular time intervals in a PL spectrometer.The corresponding emission spectra of the CPB@SiO 2 , PbN-1, and PbN-4 NCs are represented in Fig. 5a-c and the emission spectra of the remaining samples are shown in Fig. S8 (in ESI †).Among them, the SiO 2 -coated NCs are least stable under direct water contact and degraded within 10 min.Fig. 5d represents the change in emission spectra of all NCs with different time intervals after the addition of DI water.However, the polymer-coated NCs maintained their emission intensity in water for longer periods  We also performed a heat stability test of all NCs in thin-film form.The glass substrates were coated with different NCs to form separate thin-films and placed on a hot-plate (temperature B 60 1C).The PL intensities of the NCs thin-films were recorded by a PL spectrometer as a function of time.The PL spectra of the CPB@SiO 2 , PbN-1, and PbN-4 NCs thin-films with different time intervals under heating conditions are shown in Fig. 6a-c, and the PL spectra of the remaining NCs are represented in Fig. S10 (see ESI †).We observed that the heat stability of the CPB@SiO 2 NCs is the highest among all NCs.The retention in PL intensity is 95% after 32 min and 91% after 60 min of time.The high thermal stability of the SiO 2 -coated NCs is due to the lower thermal conductivity of silica. 73,74The silica shell acts as a thermal insulation layer that protects the core material against degradation in the presence of heat.Fig. 6d shows the curve representing the change in PL intensity with time.Among the polymer-coated NCs, the PbN-4 NCs also had the highest heat stability.After 32 min of time, the PL retentions for the PbN-1, PbN-2, PbN-3, PbN-4, and PbN-5 NCs are 26%, 30%, 41%, 57%, and 37%, respectively.Similarly, after 60 min the PL retentions for the PbN-1, PbN-2, PbN-3, PbN-4, and PbN-5 NCs are 13%, 15%, 30%, 42%, and 27%, respectively.These results indicate that the heat stability of the NCs increases with a certain increase in the amount of NIPAM polymer.
The instability of the perovskite NCs in water limits the detection of metal ions in an aqueous atmosphere. 36,37The comparatively higher water stability of the PbN-4 NCs encouraged us to further investigate their capability to detect metal ions in water media.To perform the ion detection experiment, we prepared different aqueous solutions containing Ni 2+ , Al 3+ , In 3+ , Co 2+ , Fe 2+ , Fe 3+ , and Cu 2+ -ions.Then these solutions were added separately in the same concentrated PbN-4 NCs solution (B5 mg mL À1 ) dispersed in DI water.The subsequent photographic images of the NCs solutions before and after the addition of different metal ion solutions are shown in Fig. 7a.The emission of the PbN-4 NCs solution is totally quenched with the addition of Cu 2+ -ion solution while the PL intensity of other NCs solutions remained almost unchanged (see Fig. 7b).The PL quenching of the NC solution in the presence of Cu 2+ -ions determines the detection capability of Cu 2+ -ions in the aqueous solution.Furthermore, we recorded the emission spectra of the PbN-4 NCs solution in the presence of different concentrations of Cu 2+ -ions varied from 0 to 412 mM.With the increase of the concentration of Cu 2+ -ions, the PL intensity of the PbN-4 NCs is monotonically decreased (see Fig. 7c and d) while the shape of the PL spectra remained unchanged or not shifted.The linear decrease in PL intensity with an increase in Cu 2+ -ions confirms the high detection capabilities towards Cu 2+ -ions in the presence of PbN-4 NCs, and the probing window is quite wide.The possible reason for quenching of PL could be due to the transfer of electrons from NCs to Cu-ions, 31 or adsorption of Cu 2+ -ions to the NC surface. 31][77] There is no spectral shift observed in the case of PbN-4 NCs after the addition of Cu 2+ -ions (see Fig. 7c).In the cation exchange process, the emission peak usually shifts either to higher/lower energies. 77So, the possibility of the cation exchange with Cu 2+ions in PbN-4 NCs is not feasible.The absorption spectra of the acceptors and emission spectra of the donors should overlap for the FRET mechanism. 78However, no such overlap is noticed between the emission spectrum of the PbN-4 NCs and absorption spectrum of copper acetate (see Fig. S11, ESI †).Generally, the electron transfer process exhibits linear emission quenching with the increase of metal-ion concentration. 31Similar emission quenching behaviour is observed in our system.This is attributed to the special d 9 electronic configuration of Cu 2+ -ions, which is favourable to accept an electron and shift to a comparatively stable d 10 electronic configuration. 79,80A schematic diagram of the charge transfer process in the presence of Cu 2+ -ions is shown in Fig. S12 (see ESI †).If the Cu 2+ -ions come into close proximity to PbN-4 NCs, they are adsorbed on the NC surface, triggering the charge transfer from the NCs to the Cu 2+ -ions.Consequently, the radiative transition from the conduction band to the valence band of the NCs is inhibited and results in PL quenching of the NCs.In order to find the detection limit of Cu 2+ -ions, we plotted the first four lowest concentrations at which the PL intensity of the NCs solution starts quenching and linear-fitted the points.To find the detection limit, we used the (3s/slope) formula. 81,82The slope and s (standard deviation) are calculated from the linear fit.Using the above formula, the lowest detection limit for the detection of Cu 2+ -ions is found to be 18.6 mM (R 2 = 0.9986) based on the slope (=0.00266) and s (=0.01658) values.These results are extremely beneficial for Cu 2+ -ion detection in natural water resources and biological systems.

Conclusions
In conclusion, we report the synthesis of CsPbBr 3 NCs via a onepot modified LARP synthetic approach under normal atmospheric conditions (relative humidity level above 80%) where the NCs were coated with different materials, such as SiO 2 , PVP, and mixed PVP-NIPAM polymers.The silica-coated NCs are made of CsPbBr 3 -Cs 4 PbBr 6 nanocomposites, while the polymercoated NCs consist of a trace amount of 2D CsPb 2 Br 5 perovskite phase along with CsPbBr 3 -Cs 4 PbBr 6 nanocomposites.The SiO 2coated NCs exhibit PL peak position at 494 nm (FWHM B 31 nm) with a maximum PLQY up to 63%.These NCs are quite big and the particle size distribution is also broad.With PVP and NIPAM polymer coating around the NCs, the PL intensity and water stability of the NCs significantly improved, but the heat stability decreased compared to SiO 2 -coated NCs.The polymer-coated NCs can be dispersible in water and maintain their PL intensity for many days.These NCs exhibit a definite shape and size with narrow size distribution that is suitable for the intake of NCs in biological cells.These NCs emit green emission in the spectral range of 513-515 nm with a maximum PLQY up to 93%.The emission FWHM of the NCs is also decreased which is beneficial for display technologies.The polymer-coated NCs exhibited better water stability as compared to the mostly used SiO 2 -coated NCs, enabling them to perform Cu 2+ -ion detection directly in an aqueous solution.These results are extremely valuable and can be used for the detection of Cu 2+ -ions in natural water resources and biological systems.

Fig. 1
Fig. 1 Stacked XRD diffraction patterns of different NCs in thin-film form.The bottom of the figure represents the standard XRD diffraction patterns of monoclinic CsPbBr 3 , rhombohedral Cs 4 PbBr 6 , and tetragonal CsPb 2 Br 5 perovskite structures.

Fig. 2 ,
Fig.2, and the corresponding particle size distributions are represented shown in Fig.S2(in ESI †).The slightly blurry surfaces around the CPB@SiO 2 NCs (see Fig.2a) appeared from the adsorption of the silica coating that is compact and has a shell thickness of a few nm.The silica shells diffuse among other silica-coated NCs and transform into bigger irregular shapes.The average particle size distribution of the CPB@SiO 2 NCs is 32 AE 7 nm, which is quite big in size.These NCs exhibit inter-planar spacing of 0.42 nm that corresponds to the (110) lattice planes of monoclinic CsPbBr 3 crystal phases.65In comparison, the polymer-coated PbN NCs (see Fig.2b-f) reveal a monodisperse and uniform particle size distribution without any noticeable inter-particle agglomeration.These PbN NCs are cubic in shape and highly crystalline in nature.A similar (110) lattice plane appeared for the PbN NCs from the monoclinic CsPbBr 3 crystal structures.The mean particle sizes of PbN-1, PbN-2, PbN-3, PbN-4, and PbN-5 NCs are around 13.47 AE 3, 13.2 AE 3, 13.1 AE 3, 10.7 AE 2, and 10.2 AE 2 nm, respectively (see Fig.S2in the ESI †).It can be seen that the particle size becomes smaller and the particle size distribution is more uniform with an increase in the amount of NIPAM polymer.It is beneficial for strong quantum confinement of charge carriers inside the NCs and hence enhances the optical properties.The UV-vis absorption spectra of all the NCs in the solution phase are shown in Fig.3a.The absorption spectrum of CPB@SiO 2 reveals a band edge peak at 507 nm originated due to 3D CsPbBr 3 perovskites and a minor peak at 320 nm that appeared from 0D Cs 4 PbBr 6 perovskites.11The PbN NCs demonstrate nearly similar absorption spectra while more intense, sharper, and well-defined absorption band edges are observed.The scattering effect of CPB@SiO 2 NCs is much higher as compared to PbN NCs because of the bigger particle sizes of CPB@SiO 2 NCs.66,67The band edge absorption peaks of PbN NCs are slightly redshifted with increasing the NIPAM polymer.The PL emission spectra of CPB@SiO 2 NCs (dispersed in ethanol) and PbN NCs (dispersed in toluene) are shown in Fig.3b.The corresponding photographic images of all NCs under a UV lamp are shown in the inset of Fig.3b.The CPB@SiO 2

Fig. 3
Fig. 3 (a) Absorption and (b) PL spectra of CPB@SiO 2 and PbN NCs as marked in the legend.The inset of (b) represents the photographic image of all NCs in the solution phase under a UV lamp.(c) FTIR spectra of CPB@SiO 2 , PbN-1, and PbN-4 NCs.(d) Photographic images of CPB@SiO 2 and PbN-4 NCs thin-films after being dipped in DI water and placed under a UV lamp.

Fig. 4
Fig. 4 PL spectra of (a) CPB@SiO 2 , (b) PbN-1, and (c) PbN-4 NCs solutions with different amounts of DI water added to the corresponding NCs solution, as shown in the legends.(d) The change in PL intensity of all NCs with the addition of water, as shown in the legend.

Fig. 5
Fig. 5 PL spectra of (a) CPB@SiO 2 , (b) PbN-1, and (c) PbN-4 NCs at a time interval of 5 min after the addition of 100 mL DI water in 100 mL of NCs solution.(d) The change in PL intensity of all NCs samples after 100 mL DI water addition as a function of time, as shown in legend.Fig. 6 Heat stability test: PL spectra of (a) CPB@SiO 2 , (b) PbN-1, and (c) PbN-4 NCs films for a period of 60 min while kept under 60 1C.(d) The changes in PL intensity of all NCs samples kept at 60 1C as a function of time, as shown in legend.

Table 1
Optical properties of different coated NCsName of the NCs PL peak position (nm) FWHM (nm) PLQY (%)