Development of inorganic-organic hybrid nanostructured material for H2O2 sensing application

An organic-inorganic hybrid nanoparticle (HNPs) composed of Sm(TTA)3Phen, a coordination compound, and NaY0.78Er0.02Yb0.20F4, an upconversion nanoparticles (UCNPs), has been developed and used for H2O2 sensing application. Herein, Sm(TTA)3Phen absorbs ultraviolet (UV) light and gives fluorescence in yellow-red-near infrared (NIR) region. Whereas, the UCNPs absorb NIR radiations (980 nm) and consequently emit in green-red region through photon upconversion process. Two important optical phenomena are observed when HNPs are simultaneously excited with UV (266 nm) and NIR (980 nm) laser radiation- (i) an energy transfer from Sm3+ to Er3+ ions, and (ii) color tunable emission from red to green, if the power of 980 nm laser is varied. Further, the material is highly competent to sense H2O2 through fluorescence quenching of Sm3+ emission in presence of H2O2. The nature of quenching is conspicuously different for different concentration/volume range of H2O2. For lower volume range, the rate of decrease of emission/excitation intensity is linear, while for higher volume range the decay in intensity is exponential. The attained minimum detection limit for H2O2 is 2 μl, which is significant for sensing applications.


Introduction
Development of intense and tunable source of light is of fundamental importance in photonics for different technological applications. This permits one to carry out various measurements without disturbing the source and experimental setup. Such tunable light sources are frequently used in bio-medical sensing and industrial applications also [1][2][3][4][5][6]. A large variety of dye lasers, both in pulse and continuous mode, are already well known sources for tunable emission. However, these sources have limited tunability of only few Angstroms. Diode lasers also are the sources which are used for tunable emission but again they are tunable only upto 100 Angstroms. In this work, we have reported a hybrid material which gives us tunable radiation in between 500-700 nm spectral region both through normal fluoresce and photon upconversion process. Rare-earth (REs) are the active material for multi-color luminescence.
The unique and specific property of some trivalent REs (Er 3+ , Tm 3+ , Pr 3+ , Tb 3+ and Ho 3+ ) is to absorb two or more low energy photons via the involvement of real metastable states and thereafter consequently emit high energy photons. This process is termed as upconversion (UC) process [7][8][9][10][11][12][13][14]. The UC process is only feasible in REs. In addition to this, many of the REs have energy levels to absorb near infrared (NIR) photons perfectly matching with 980 nm diode laser [15][16][17][18]. One potential element in these REs is Yb 3+ ion, which absorbs 980 nm photons very efficiently (more than ten times as compared to any other REs) due to large absorption cross-section. It may transfer its excitation energy to the resonant energy levels of other REs (e.g. Er 3+ , Tm 3+ , Pr 3+ , Tb 3+ and Ho 3+ ) and act as a sensitizer [19][20][21]. Thus, Yb 3+ ions are used as co-dopant with other REs for OPEN ACCESS RECEIVED 10 January 2020 Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. efficient UC luminescence. Further, optical properties of REs doped materials depend on local crystal field symmetry and hence on crystal structure, particle size and phase of the host material also [22][23][24][25][26]. In this work, NaY 0.78 Er 0.02 Yb 0.20 F 4 is chosen for UC emission due to its highest quantum yield reported so far in the literature [27]. The emitted radiation of the active ion Er 3+ lies mainly in green-red region.
REs based β-diketonates coordination compounds absorb ultraviolet (UV) and blue photons extensively through singlet state electronic transitions as due to presence of large number of π-electrons in the organic ligands. The absorbed energy of ligand in singlet state is transferred to its triplet states through intercross relaxation process. Thereafter, in the next step, it is resonantly transferred to the resonant energy states of REs, from where, intense and sharp emissions of REs are observed. This process of energy transfer is termed as sequential or cascade energy transfer [28][29][30][31][32]. In this work, a β-diketonate Sm(TTA) 3 Phen (STP) is chosen as it gives intense red emission of Sm 3+ ion through normal photoluminescence process. The TTA and Phen ligands absorb UV-blue radiation via their singlet states (S 1 and S 2 ) and transfer it to their triplet states and finally to resonant and long lived energy states of Sm 3+ . The excited Sm 3+ gives one transition in green region (562 nm) and three transitions (609 nm, 646 nm and 714 nm) in yellow-red region in which emission peaking at 646 nm exhibits the highest intensity. Thus, the developed hybrid material may give emission ranging from green region to extreme red region (520-720 nm) under different source of excitations.
The unique photo-physical properties of HNPs further motivate us to extend this work for sensing of reactive oxygen species (ROS), particularly hydrogen peroxide (H 2 O 2 ). Hydrogen peroxide (H 2 O 2 ) is one of the principal members of reactive oxygen species (ROS) and affects biological systems significantly, used in industries, pharmaceuticals and other fields, etc [33][34][35][36][37][38][39]. H 2 O 2 is also present in atmosphere in small content. In addition to this, H 2 O 2 is also a byproduct of the reactions catalyzed by large number of oxidase enzymes, where the oxidase enzyme converts H 2 O 2 into water and oxygen [37]. In past three decades, different elegant spectroscopic, electrochemical and enzymatic approach based techniques have been used for the detection of H 2 O 2 [38][39][40][41][42][43][44].
In this work, the H 2 O 2 sensing application has been carried out using spectroscopic (fluorescence) technique, in which the fluorescence intensity is measured and plotted as a function of different volume/content of H 2 O 2 . The H 2 O 2 volume dependent variation in luminescence intensity, linear for low volume and exponential for high volume, is unique by any fluorophore and first time reported in this work.

Synthesis of Sm(TTA) 3 Phen coordination compound
The synthesis of STP was carried out using solution cast technique [29]. The SmCl 3 .6H 2 O was prepared by dissolving samarium oxide into minimal amount of concentrated hydrochloric acid. TTA (6 mmoles), Phen (2 mmoles) and 8 ml of 1N solution of NaOH were dissolved in ethanol and a homogeneous solution was prepared. (2mmoles) of SmCl 3 .6H 2 O was dissolved in 10 ml of distilled water and then added to the former solution with a constant stirring for better reaction. A white color precipitate of Sm(TTA) 3 Phen complex was formed. The precipitate was further washed with distilled water for four times and dried in oven at 40°C.

Synthesis of hybrid nanophosphor (HNPs)
The Sm(TTA) 3 Phen and NaY 0.78 Er 0.02 Yb 0.20 F 4 were mixed in cyclohexane in equal molar ratio under a constant stirring for five hours. The mixture is then centrifuged for 1 h in order to get well developed HNPs. The HNPs was then dried in oven at 40°C for ten hours.

Characterization
X-ray diffraction (XRD) patterns of UCNPs, STP, and HNPs were measured using X-ray diffractometer (Model No: Miniflex-II, Rigaku, Japan) with CuK α radiation λ=1.5406Å. The transmission electron microscope (TEM) and the high resolution TEM images were recorded using an FEI, Tecnai G 2 , S-Twin instrument operating at 200 KV. Fourier transform infrared (FTIR) spectra were recorded using Perkin-Elmer frontier FTIR spectrometer-35 in the spectral range 400-4000 cm −1 . The UV-vis-NIR absorption spectra of these samples were recorded in the range 200-1100 nm using Perkin Elmer, Lambda 750 spectrophotometer. Photoluminescence excitation, emission, lifetime measurements were performed by fluorescence spectrometer (Model no: FL-3-11 Horiba Jobin Yvon) equipped with 25 W pulsed xenon (Xe) lamp, 450 W CW Xe lamp and PMT detector (model No: FL-1073). The UC emission spectra were recorded by using 980 nm (CW, 2W, power tunable) diode laser as an excitation source and a dispersing monochromator (Model no: iHR320, Horiba Jobin Yvon) attached with a PMT (Model no.1424M) as a detector.

Results and discussion
3.1. Transmission electron microscopy (TEM) and x-ray diffraction (XRD) analysis TEM measurements of UCNPs and HNPs are carried out to observe the microstructure and particle size. Figure 1(a) shows the TEM image of UCNPs at 100 nm scale bar. Average size of the particles is found in the range of 10-15 nm. Figure 1(b) shows the TEM image of HNPs and it clearly depicts that, the UCNPs are fairly dispersed in STP and forms a sheet like structure. The inset of figure 1(a) shows the selected area electron diffraction (SAED) pattern of UCNPs. The SAED pattern shows four rings and these rings corresponds to (1,1,1), (2,0,0), (2,2,0) and (3,1,1) planes of NaYF 4 crystal in face centered cubic phase. Phase of the NPs have been further verified using XRD analysis also. Figure S1 is available online at stacks.iop.org/MRX/7/056201/mmedia shows the XRD pattern of UCNPs, STP and HNPs. The XRD pattern of UCNPs matches well with the face centered cubic phase of NaYF 4 crystal with lattice parameters a=b=c=5.448 Å (JCPDF file no. 06-0342). The calculated d-spacing from SAED pattern matches well for the inter-planner separation between the (1,1,1), (2,0,0), (2,2,0) and (3,1,1) planes with JCPDF file no. (06-0342). Crystallite size has also been calculated by using Debye Scherer's formula [47,48]. Average crystallite size is found to be 7.9 nm which is in accordance with TEM measurements.

Absorption spectra of solid UCNPs, STP and HNPs
Absorption spectra of solid UCNPs, STP and HNPs are recorded in the range from 200-1000 nm and are shown in figure 2. The absorption spectrum of UCNPs shows no peaks in UV and visible region, although a weak absorption band is observed in NIR region around 980 nm (as shown in the inset of figure 2). The host absorption band due to NaYF 4 lies at higher energy while the peaks due to Er 3+ do not appear due to low absorption cross-section. On the other hand, the absorption spectrum of STP shows two intense absorption bands peaking at 273 nm and 390 nm. The intense absorption band at 390 nm is due to π→π * electronic transition (S 0 →S 1 ) of TTA while broad absorption band at 273 nm probably arises due to overlapping of π→π * electronic transition (S 0 →S 1 ) of phen and n→π * electronic transition (S 0 →S 2 ) of TTA [28].
The absorption spectrum of HNPs contains peaks both due to UCNPs and STP, and shows strong UV absorption due to STP and weak NIR absorption due to UCNPs. The absorption intensity of bands remains almost unaltered as compared with STP and UCNPs. This also suggests that, both the constituent in HNPs are not interacting strongly. So, it is expected that the constituent particles of HNPs are entangled through weak forces, like Vander wall interaction, similar to composite materials [45,46]. Figure 3(a) shows emission spectra of STP and HNPs under 390 nm excitations. The emission spectrum of STP on excitation with 390 nm radiation shows emission bands at 562, 609, 646 and 706 nm which are assigned due to 4 G 5/2 → 6 H 5/2, 4 G 5/2 → 6 H 7/2 , 4 G 5/2 → 6 H 9/2 and 4 G 5/2 → 6 H 11/2 transitions of Sm 3+ ion, respectively. Among all these transitions, 4 G 5/2 → 6 H 9/2 transition peaking at 646 nm is most intense and sharp which is responsible for the characteristic red emission of STP complex [29]. The excited Sm 3+ ions in different states relax down to 4 G 5/2 metastable state by non-radiative relaxation process. The emission intensity is maximum under the 390 nm excitation due to combined effect of energy transfer from ligand to Sm 3+ ion and a direct excitation of Sm 3+ ions. The spectral lines also show Stark's components which indicate that Sm 3+ ions are surrounded by ligand. The similar emission spectrum is observed in HNPs also, only emission intensity varies slightly.

Photo-physics, decay dynamics and energy transfer in coordination compound STP
The decay curve of 4 G 5/2 → 6 H 9/2 transition corresponding to 390 nm excitation for STP and HNPs are shown in figures 3(b), (c). The lifetime of 4 G 5/2 levels of Sm 3+ ion in STP and in HNPs are found to be 66 μs and 71μs, respectively. The lifetime in HNPs is slightly higher. It may be due to weak Vander wall interaction and reduction in high frequency vibrational bands ( figure S2 shows the FTIR spectra of UCNPs, STP and HNPs. The detailed analysis is mentioned in supplementary section). The emission spectra and the lifetime can be explained by using the partial energy level diagram shown in figure 4. In coordination compounds, the energy transfer from organic ligands to lanthanide ions is termed as 'antenna effect'.
In the present case, energy is transferred from TTA and phen ligands to Sm 3+ ions [28]. It is obvious from figure 4 that the UV photons are initially absorbed by TTA and Phen through their singlet electronic states. The energy transfer from ligand to Sm 3+ ions may occur in different ways-(i) electronically excited level of phen (S 1 ) is very close to S 2 state of TTA. Thus a transfer of energy from phen singlet S 1 to TTA singlet S 2 may occur. The energy from S 2 level of TTA subsequently relaxes to S 1 energy level of TTA via internal conversion (IC). The TTA then may transfer its energy to Sm 3+ ions (via singlet level or triplet level of TTA to Sm 3+ ion), (ii) It is also possible that, phen is excited to singlet S 1 via S 0 →S 1 absorption and then relaxes nonradiatively to its triplet   state. The triplet level of phen transfer its energy to triplet level of TTA and finally to Sm 3+ ions. The ligand TTA excited to S 1 may also transfer its energy to triplet of TTA, via inter-crossing system (ISC). Then the TTA triplet level finally transfers the energy to Sm 3+ ions. Thereon the different transitions of Sm 3+ are observed.

Upconversion emission
The UC emission has been monitored by using 2W tunable continuous wave 980 nm diode laser as a source of excitation. It is found that STP gives no UC emission. On the other hand, UCNPs and HNPs give intense UC emission. The upconversion emission spectra of UCNPs and HNPs are shown in figure 5(a). The emission bands are observed at 521, 540 and 649 nm corresponding to 2 H 11/2 → 4 I 15/2, 4 S 3/2 → 4 I 15/2 and 4 F 9/2 → 4 I 15/2 transitions of Er 3+ ions, respectively. The red emission at 650 nm is relatively weak than the green emission bands. It is important to notice that, all the emission bands show Stark components which suggest a high electric field of host material on the Er 3+ ions. The energy levels of the sensitizer Yb 3+ , activator Er 3+ and upconversion mechanism is shown in figure 5(b).
There are two processes responsible for UC emission, excited state absorption (ESA) of Er 3+ and energy transfer from Yb 3+ to Er 3+ ion. In ESA process, Er 3+ ions absorb pump photons (980 nm) and are promoted to excited state 4 I 11/2 . Some of the excited Er 3+ ions in 4 I 11/2 level may relax and populate low lying 4 I 13/2 level. Now, population in 4 I 13/2 , 4 I 11/2 absorbs another photon and are promoted to 4 F 9/2 and 4 F 7/2 levels in presence of laser excitation. The ions in the 4 F 7/2 excited state further relax nonradiatively to the metastable states 2 H 11/2 and 4 S 3/2 which ultimately decay to ground state and gives green emission. Similarly, the red transition is observed from 4 F 9/2 level. The levels 4 I 11/2 , 4 F 7/2 are also populated from an energy transfer of Yb 3+ to Er 3+ , as shown on figure.
The number of photons involved in a particular UC luminescence process can be given a.s [49,50].  HNPs, the value of n is 2.3 and 2.2 for 520 nm and 650 nm emissions, respectively. This also supports our proposed mechanism of two photon upconversion process for green and red emission.

Simultaneous excitation of Sm 3+ and Er 3+
The emission spectrum of HNPs was monitored by focusing two excitation (266 nm and 980 nm, both by laser source) collinearly on the sample simultaneously also. Figure 6 shows the emission spectra of the HNPs monitored by exciting the sample with 980 nm and 266 nm individually and then simultaneously. The UC spectrum (λ exc =980 nm) gives characteristic emission bands of Er 3+ as explained above. Similarly, photoluminescence emission spectrum (λ exc =266 nm) gives characteristic emission bands of Sm 3+ . However, the emission spectra monitored with 266 nm+980 nm excitations show two new optical effects-(i) an energy transfer from Sm 3+ to Er 3+ , and (ii) a strong and broad red emission at about 650 nm. The first effect is based on the fact that, the emission intensity of Er 3+ is increasing while the emission intensity of Sm 3+ is decreasing upon excitation of HNPs with two lasers simultaneously. Particularly, emission intensity of 4 S 3/2 → 4 I 15/2 transition of Er 3+ (at 540 nm) increases while emission intensities of 4 G 5/2 → 6 H 5/2 and 4 G 5/2 → 6 H 7/2 transitions of Sm 3+ (at 562 nm and 609 nm, respectively) decrease. In fact emission intensity due to 4 G 5/2 → 6 H 5/2 transition (at 562 nm) almost diminishes. The 4 G 7/2 state of Sm 3+ is close with the 4 F 7/2 level of Er 3+ , thus the energy transfer from Sm 3+ to Er 3+ seems possible (as shown in figure 7). Further, the second effect is observed in the view of combined intensity of emission due to 4 G 5/2 → 6 H 9/2 transition of Sm 3+ and 4 F 9/2 → 4 I 15/2 transition of Er 3+ . Thus, the strong red emission is due to the overlapped emission band of both Sm 3+ and Er 3+ under dual excitation.
The hybrid material promises the color tunability in the range from 520 nm to 660 nm with laser excitation power. The CIE diagram shown in figure 8 indicates that the HNPs shows the pure red emission under 266 nm exciation. Now, if we increase the power of 980 nm diode laser (keeping 266 nm laser power fix) the HNPs shows the tunable colour from pure red, yellowish to pure green color [51,52]. Variation of color coordinate with different laser power is given in table 1.

Application of HNPs as sensor for H 2 O 2
In order to realize a sensor for H 2 O 2 , firstly, the HNPs were dissolved in cyclohexane keeping concentration of solution maintained at 10 -5 M. Then, different volume/content (starting from 2 μl) of H 2 O 2 (in step of 2μl, upto 58μl) were added to 10 ml of HNPs solution. Presence of H 2 O 2 in HNPs highly affects both excitation as well as emission intensity of Sm 3+ and thus both may be used as parameter to probe the sensitivity. Figure 9 shows excitation spectra (λ ems =645 nm) of HNPs as a function of H 2 O 2 content. It clearly depicts that, intensity of the excitation band decreases on increasing the contents of H 2 O 2 . Plot of excitation intensity versus H 2 O 2 contents shows two clearly distinct patterns. The first pattern is observed for upto 28 μl content of H 2 O 2 ( figure 9(a)) that shows a minimum of 2 μl of H 2 O 2 content can be sensed with resolved excitation intensity. The rate of intensity drop in this regime is slow and it shows almost a linear behavior ( figure 9(c)). The second type of pattern is observed for the H 2 O 2 content beyond 28 μl upto 58 μl ( figure 9(b)). The rate of intensity drop in this content range is very fast and shows exponential decay behavior ( figure 9(d)).
The measurement was repeated and the result remains unaltered. The sensitivity (s) of linear region can be calculated using the slope of the straight line and can be expressed as- Where, ΔI is the change in intensity, I is the intensity at zero μl content of H 2 O 2 and Δc is the change in the volume of H 2 O 2 . Sensitivity for linear part thus calculated is found to be 2.95% signal decrease per μl of H 2 O 2 addition, which is quite high for sensing purpose.   , it causes fast non-radiative relaxations, resulting in a decrease in radiative emission. As the content of H 2 O 2 increases water content also increases that causes non-radiative relaxations and the radiative emission disappears. The second mechanism proposes that, the presence of H 2 O 2 changes the environmental symmetry around Sm 3+ ions and therefore emission intensity may also decrease.
The plot of emission intensity versus H 2 O 2 content (figures 10(b)-(g)) for all the three emission peaks (562 nm, 602 nm and 645 nm) of Sm 3+ also shows two clearly distinct patterns as it was observed in case of excitation. At lower contents (<22 μl), it shows a linear decay behavior while for the higher contents it shows an exponential decay behavior. The sensitivity for 562 nm, 602 nm and 645 nm peaks calculated using equation (1) for lower content region is 2.89%, 3.47% and 3.55% signal reduction per μl of H 2 O 2 , respectively. Thus the

Conclusion
Photophysical properties, energy transfer studies and H 2 O 2 sensing application have carried out using an organic-inorganic hybrid nanoparticles (HNPs) composed of Sm(TTA) 3 Phen and NaY 0.78 Er 0.02 Yb 0.20 F 4 . It is a dual mode emitting (both through photoluminescence and photon upconversion) and color tunable material. The HNPs shows an energy transfer from Sm 3+ of coordination compound to Er 3+ of inorganic NPs on exciting the material with 266 nm and 980 nm lasers simultaneously. Presence of H 2 O 2 in the solution of HNPs (in cyclohexane) quenches the excitation and emission intensity of Sm 3+ significantly. This property has been used for sensing of H 2 O 2 with its specificity for lower and higher content of H 2 O 2 . For lower contents, the rate of decrease in emission (as well as excitation) is slow and shows a linear behavior, while it is very fast for higher contents and shows exponential behavior. The sensitivity has also been calculated for the linear region which is reasonably good. Such low cost HNPs may find vast applications in different areas ranging from sensors to photonics to biomedical.