Improvement of Photostability and NIR Activity of Cyanine Dye

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A: Kinetics, Dynamics, Photochemistry, and Excited States

Improvement of Photostability and NIR Activity of Cyanine Dye Through Nanohybrid Formation: Key Information from Ultrafast Dynamical Studies Arpan Bera, Damayanti Bagchi, and Samir Kumar Pal J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b04100 • Publication Date (Web): 11 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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The Journal of Physical Chemistry

Improvement of Photostability and NIR Activity of Cyanine Dye through Nanohybrid Formation: Key Information from Ultrafast Dynamical Studies

Arpan Bera, Damayanti Bagchi, Samir Kumar Pal*1 1Department

of Chemical, Biological and Macromolecular Sciences, S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700 106, India

*Corresponding

Author E-mail: [email protected] Telephone: +91 033 2335 5706-08 Fax: +91 033 2335 3477

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Abstract NIR light harvesting has enormous importance for different potential applications in the modern era of research. Some NIR cyanine dyes such as IR820 have achieved great success in energy harvesting and cancer therapy. However, their action is limited for low photostability, considerable thermal degradation, short circulation times and nonspecific bio-distribution. Our present study is an attempt to overcome such limitations by attaching a model cyanine dye IR820 with ZnO nanoparticles. We have prepared IR820ZnO nanohybrid and characterized it using microscopic and optical spectroscopic tools. Thermogravimetric analysis has shown greater thermal stability of IR820-ZnO nanohybrid compared to free dye. We have explored the enhancement in photostability of IR820 upon nanohybrid formation. We have detected generation of photo-induced reactive oxygen species (ROS) such as super oxide, singlet oxygen etc. using appropriate molecular probes. The formation of IR820-ZnO nanohybrid reduces production of photo-induced singlet oxygen. However, it depicts an alternate trend in overall ROS formation (increases total ROS) under red light illumination. To correlate, enhanced photostability of IR820 on ZnO surface, we have explored excited state dynamical processes at the interface in nanohybrid. We have illustrated photoinduced excited state electron transfer process from LUMO of IR820 to conduction band of ZnO. This photoelectron transfer process enhances the production of ROS, decreases the formation of singlet oxygen that altogether leads to improvement in photostability and overall activity. A quencher of singlet oxygen sodium azide (NaN3) was used to further confirm direct association of singlet oxygen generation with photostability issue of IR820. Also, ZnO is able to deliver the dye selectively in acidic environment that suggest its diseased site specific targeted activity. Our results provide a promising improvement for potential use of IR820 through formation of nanohybrid that could be translated for other NIR cyanine dyes. Keywords: NIR cyanine dyes, photostability, thermal stability, reactive oxygen species (ROS), singlet oxygen, superoxide species, photo-induced electron transfer and targeted drug delivery platform.


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1. Introduction Light harvesting materials that capture solar photons and convert them for light mediated applications is considered to be one of the ground breaking research topics of present day. Light harvesting assemblies provide wide range of application as in dye sensitized solar cells, photodynamic therapy (PDT), photocatalysis, bio-imaging, hydrogen evolution, toxic elements sensing etc.1 Moreover, near infrared (NIR) light harvesting from solar spectrum is particularly significant for photocatalysis, bio-imaging and photodynamic therapy.2,

3

NIR

(range from 650 to 1350 nm) radiation is more promising for photodynamic therapy due to its large penetration depth in tissue.4 Light harvesting system can be classified into organic dyes, metal-ligand complexes, inorganic semiconductors, quantum dots, organic-inorganic nanohybrids, bio-molecular assemblies etc.5,

6

TiO2 and ZnO are the most promising

semiconductors which have been used in different way of light harvesting from many years.7, 8

Among the long list of light harvesting materials, dyes are the most widely used system till

date. Basically, dyes are less toxic than inorganic materials and they are capable of efficient light harvesting as they have very high molar extinction co-efficient. Many NIR dyes have been used as therapeutics due to their photodynamic action, photo thermal effect and suitable efficiency of singlet oxygen generation that can kill infected cells.9, 10 Recently, in case of dye sensitized solar cells (DSSC) also, there is a growing interest for NIR sensitization.11, 12 Also, some dyes have been used in photocatalytic hydrogen evolution and organic pollutant degradation.13 In last few years, cyanine dyes have been most important NIR dye for DSSC and PDT application.14,

15

There are also some reports on their photocatalytic activity.16 They show

different photo-physical properties depending on their various structures.17 One of the disabilities of these NIR dyes is poor photostability under NIR radiation.18, 19 Photostability of cyanine dyes is not suitable for their potential use. The photobleaching pathway of cyanine 3 ACS Paragon Plus Environment


dyes is initiated by their first triplet excited (T1) state. Upon photon absorption, electrons transit from ground state (S0) to first singlet excited (S1) state. Besides fluorescence mediated relaxation, electrons can shift to T1 state through intersystem crossing (ISC). This long lived T1 state is quenched by molecular oxygen to generate singlet oxygen.20 Basically, production of efficient singlet oxygen21, 22 by cyanine dyes leads to oxidation of the dye (1O2, being low lying LUMO is a very strong oxidising agent23-25) that is responsible for its photodegradation.20, 26 Moreover, cyanine dyes contains exo-cyclic C-C double bonds. The singlet oxygen causes 1, 4-addition reaction to cis-diene or 1, 2-addition reaction to alkene to form a dioxene structure or 1, 2-dioxitane structure respectively.27 Therefore, improvement of photostability is required for potential applicability of cyanine dyes. Besides low photostability, thermal degradation of cyanine dyes have been considered as an issue for its real applications.28 To control the problem of low photostability of these cyanine dyes, few methods have been reported including encapsulation in the negatively charged copolymer,19 non-covalent encapsulation inside silica nanoparticles29 or doping with silica nanoparticles30 and cucurbituril encapsulation.31 Another efficient strategy to improve the photostability of cyanine dye is the introduction of some electron deficient substituent, which can reduce the reactivity of dye towards singlet oxygen.20, 26, 32 The most general way for the enhancement of photostability is conjugation with triplet state quencher such as cyclooctatetraene (COT).33 However, for all these methods complicated costly synthetic strategies are required. In our present study, we report a new approach that focuses on the photostability of a very well-known cyanine dye IR820. IR820 has been widely used in photodynamic therapy due to its ability of singlet oxygen generation21 under NIR radiation. It has been used in near infrared room temperature photovoltaic photon detectors34 and to enhance up-conversion luminescence.35, 36 Also, like others cyanine dyes it has an issue of photobleaching.37 We have 4 ACS Paragon Plus Environment

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chosen semiconductor zinc oxide nanoparticles (ZnO NPs, band gap ~ 3.3 eV) and developed IR820 functionalised nanohybrid IR820-ZnO. We specifically select ZnO NPs due to their biocompatibility, low cost and ease of synthesis.38 IR820 dyes have been attached on ZnO surface, which has been confirmed by optical spectroscopy tools. We have explored a significant enhancement of photostability of IR820 dye on ZnO surface as compared to free IR820. We have monitored the generation of ROS under red light using DCFH (dichlorofluorescin) indicator. The rate of ROS generation is enhanced for IR820-ZnO as compared to that of IR820. To evaluate the nature of ROS, we have performed SOSGR (singlet oxygen sensor green reagent) assay for singlet oxygen detection and luminol chemiluminescence for superoxide detection. We have also explored the photoinduced excited-state electron transfer process from dye to ZnO in the nanohybrid under red light excitation, which can result from the molecular-level interaction between two moieties. A singlet oxygen quencher (NaN3) was used to analyze the singlet oxygen mediated photodegradation of IR820 dye and its improvement of photostability on the surface of ZnO. In order to check thermal stability of IR820-ZnO nanohybrid, we have done thermogravimetric analysis (TGA) of IR820-ZnO, IR820 and ZnO. Furthermore, we have checked pH responsive dissolution and precipitation of nanohybrid to verify the capability of potential biological application. Overall, our present study is an attempt for the improvement of photostability and activity of NIR cyanine dye. 2. Experimental details 2.1. Material In our study, we have employed all the chemicals of analytical grade and used without any further purification. ZnO nanoparticles (approximately of 20 nm), IR820 were purchased from Sigma-Aldrich. DMSO (dimethyl sulfoxide) from Merck and ultra-pure water of Milipore 5 ACS Paragon Plus Environment


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system (18.2 MΩ cm) were used. SOSGR (singlet oxygen sensor green reagent) were purchased

from

molecular

probes,

Thermo

Fisher

Scientific.

DCFH-DA

(dichlorofluoresceindiacetate) was obtained from Calbiochem. NaN3 were purchased from Sigma-Aldrich. 2.2. Synthesis of nanohybrid The surface of ZnO NPs were functionalised by IR820 dye by adding 12 mg of ZnO NPs into 10 ml of IR820 solution (1 µM solution in DMSO) with continuous stirring at room temperature, for 12 h. After 12 h, the solution was centrifuged for 25 min and the clear supernatants containing the free dyes were discarded. Then, the nanohybrids were washed with DMSO for three times to remove free dyes. After washing, the nanohybrid was dried in the oven for 5 hr under 150˚C. 2.3. Characterization techniques For TEM analysis, diluted solutions of ZnO NPs in DMSO were spread over carbon-coated cupper grid. X-ray diffraction patterns of the sample were measured by producing a scanning rate of 0.02° S-1 in the 2θ range from 20° to 80° using a PANalytical XPERTPRO diffractometer equipped with Cu Kα radiation (at 40 mA and 40 kV). Absorption measurements were carried out through a shimadzu spectrophotometer (UV-2600), and a HORIBA Fluorolog was used to measure steady state emission. A time-correlated single-photon counting setup from Edinburgh Instruments was used for measuring time-resolved photoluminescence. The emission was detected as 842 nm while the excitation was at 633 nm. The measured IRF (instrument response function) was 98 ps. The nonlinear least-squares procedure was done for the fitting of the fluorescence transients.39 A red LED source was used ( max = 640 nm and power = 3 mW/cm2) for all light-activated experiments.


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2.4. Measurement of H-aggregation For the measurement of H-aggregation, different concentrations of IR820-ZnO nanohybrid (the concentration of IR820 in the nanohybrid was 0.575µM, 1.15µM and 1.725µM respectively) and IR820 (0.575 µM, 1.15µM and 1.725µM) have been dissolved in water and the absorption spectra were monitored. 2.5. Photostability measurement For photostability measurement, we have used IR820-ZnO nanohybrid (the concentration of IR820 in the nanohybrid is 1.77 µM) and IR820 (1.77 µM) in DMSO. In order to check NaN3 mediated photostability, we have used the mixture of DMSO and H2O (DMSO: H2O = 2:3) where the concentration of IR820 is 1.77 µM and the concentration of IR820 in the nanohybrid is 1.77 µM. 2.6. Measurement of ROS DCFH was prepared by de-esterification reaction of DCFH-DA.40 We have used IR820 (0.7 µM), IR820-ZnO nanohybrid (the concentration of IR820 in the nanohybrid is 0.7 µM) and ZnO (maintaining the O.D at 370 nm same with the IR820-ZnO nanohybrid which contains 0.7 µM of IR820) for ROS measurement. To detect the singlet oxygen generation, the SOSGR from molecular probes was used. IR820 (0.7 µM) and IR820-ZnO nanohybrid (the concentration of IR820 in the nanohybrid is 0.7 µM) were used for the SOSGR assay. Furthermore, to isolate the superoxide species, the basic (pH 12) solution of luminol (to the aqueous solution of luminol, NaOH was added) was used. The concentration of IR820 in the nanohybrid and the free IR820 were fixed to 0.725 µM for this luminol chemiluminiscence experiment. Singlet oxygen scavenger NaN3 was used for confirming the formation of singlet oxygen. IR820 (1.45 µM) and IR820-ZnO nanohybrid (the concentration of IR820 in the nanohybrid is 1.45 µM) were used for the final SOSGR assay in presence and absence of NaN3. 7 ACS Paragon Plus Environment


2.7. Dissolution and Precipitation For dissolution and precipitation experiment we have used IR820-ZnO nanohybrid in water at pH 5 and pH 7. The pH was maintained by using the buffers of acetate buffer (pH 5) and phosphate buffer (pH 7) respectively. The absorbance of ZnO at 370 nm is monitored with variable time window. 3. Results and Discussion High-resolution transmission electron microscopic image of ZnO nanoparticles is depicted in Fig. 1a. An interplanar distance of 0.28 nm has been found from the lattice fringes of ZnO nanoparticles, which corresponds to the (100) crystal planes.41 The average diameter of ZnO nanoparticles is ~20 nm. Fig.1b represents the chemical structure of IR820 dye. Fig. 1c is the schematic representation of IR820-ZnO nanohybrid formation. The conjugation between IR820 dye and defect sites of ZnO crystal leads to formation of nanohybrid. To check the crystal structural stability of the nanohybrid, we have performed the XRD analysis. Fig. 1d delineates the characteristic X-ray diffraction of ZnO nanoparticles (pink) and IR820-ZnO nanohybrid (blue).The diffraction pattern of ZnO depicts wurtzite42 crystal structure that remain unaltered after conjugation with IR820 dye. Thus, we infer that the crystal structure of ZnO remains intact upon functionalization with IR820. Fig. 2a represents the absorption spectra of IR820 dye and IR820-ZnO nanohybrids in methanol. The characteristic peak of IR820 is observed at 820 nm with a shoulder at 752 nm. The main peak at 820 nm corresponds to the 0→0 transition where as the shoulder at 752 nm is the 0→1 vibronic sub-band.43, 44 In case of IR820-ZnO nanohybrids, the characteristic peaks of both IR820 and ZnO are observed. However, there is no significant shift of absorption maxima of IR820 is observed in the nanohybrid. To check the solvent dependent absorption characteristics of IR820 dye, we have checked absorbance of IR820 in water in a concentration


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dependent manner. As shown in Fig. 2b, an extra peak at 686 nm is observed in presence of water and it enhances upon increase in concentration of IR820 in H2O. This peak inversion indicates H-aggregation and the peak at 686 nm represents higher order H-aggregation form of IR820.45 The H-aggregation of IR820 in water is restricted on ZnO surface when concentration is increased. The extra peak at 686 nm (which designates the H-aggregation of the dye) of IR820 in water is not clearly observed in the nanohybrid at the same concentration of IR820 (Fig. 2c). The attachment of dye on ZnO surface enhances the rigidity of IR820 dye which might be responsible to protect the dye structure from H-aggregation.46 There is a visible colour change after functionalization of ZnO, as shown in the inset of Fig 2d that depicts the colour of ZnO nanoparticles changes from white to grey upon dye attachment. In steady state emission spectra (Fig. 2d), the peak near 834 nm is clearly observed for both dye and nanohybrid (ex = 800 nm) in methanol. The significant quenching in emission intensity for IR820-ZnO nanohybrid indicates the presence of excited state non-radiative process. After optical characterization, we have studied the thermal stability of the nanohybrid by thermogravimetric analysis (TGA). Fig. 3a depicts the TG curves of IR820, ZnO and IR820ZnO. The weight loss of IR820 dye within 30-700˚C is approximately 58% and the pure ZnO NPs shows negligible degradation within this temperature range. IR820-ZnO nanohybrid shows negligible weight loss within the range of 30-700˚C of approximately 5%. As, the IR820 dye has melting point greater than 300 C, the maximum weight loss of IR820 is observed within the range of 300-500 C. Hence, the mass loss of the nanohybrid as compared to that of ZnO within 375-500˚C implies the presence of IR820 dye. This analysis shows that IR820 retains thermal stability at temperature range 30-700°C after nanohybrid formation. Further, we have checked the photo-stability of the dye upon hybrid formation. Fig. 3b represents the time dependent photodegradation of IR820 and IR820-ZnO nanohybrid. The experiment was performed in DMSO under red light and the change in absorption peak of IR820 at 834 nm is 9 ACS Paragon Plus Environment


monitored with variable time window. The rate of degradation of IR820 is higher as compared to that of IR820-ZnO nanohybrid under red light irradiation. We have estimated the degradation percentage to be 26.5% in case of IR820 and 15.8% for nanohybrid after 20 minutes of experimental time window. After evaluating the improvement in thermal and photo stability of IR820 dye upon hybrid formation using ZnO NPs, we illustrate in vitro photo-induced ROS generation using well-known marker DCFH (dichlorofluorescein). DCFH is oxidised to fluorescent DCF in presence of ROS (reactive oxygen species) and exhibits a strong emission near 522 nm upon 488 nm excitation.38 Thus, the enhancement of emission intensity at 522 nm signifies presence of greater ROS level. The experiment was done under dark for 10 minutes and under irradiation of red light for 30 minutes. In dark, there is no change in emission intensity at 522 nm. This suggests no activity of the nanohybrid under dark condition. With increase in light exposure time, the greater enhancement of emission intensity (@522 nm) was observed for IR820-ZnO as compared to that of IR820 and ZnO. Thus, the red light triggers the efficient generation of ROS for IR820-ZnO nanohybrid (Fig. 4a). The trend suggests improved applicability of nanohybrid compared to dye systems. To investigate the nature of generated ROS, we have further done SOSGR assay for singlet oxygen detection and luminol chemiluminescence reaction for super oxide detection. Singlet oxygen sensor green reagent (SOSGR) is a wellknown singlet oxygen marker which is selectively oxidised by singlet oxygen and converted to SOSGR endoperoxide (SOSGR-EP). SOSGR-EP provides green fluorescence in the range of 525-536 nm.38 The enhancement of singlet oxygen during our experiment was monitored at 530 nm. After red light illumination for 18 minutes greater enhancement of emission intensity (@530 nm) was observed for IR820 as compared to that of IR820-ZnO (Fig. 4b). To detect superoxide, we have further done luminol chemiluminescence. More specifically, luminol is oxidised by superoxide (O2.-) to produce chemiluminescence.47 The rate of superoxide 10 ACS Paragon Plus Environment

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generation was studied by monitoring the emission peak of chemiluminescence at 425 nm.48 As shown in Fig. 4c, chemiluminescence is observed for IR820-ZnO nanohybrid after red light irradiation for 15 minutes. For IR820-ZnO, the higher emission intensity at 425 nm as compared to that of IR820 indicates the greater generation of superoxide generation than IR820. We have observed from Fig. 4a, 4b and 4c that attachment of IR820 dye on ZnO nanoparticles, increases overall ROS production with reduction of singlet oxygen formation as compared to bare dye system. The fabricated nanohybrid is more photostable and more capable in producing overall reactive oxygen species but less in terms of singlet oxygen generation. We anticipate some relationship between these two aspects and further perform picosecond resolved studies to check excited state charge transfer dynamics. Fig. 5a represents the fluorescence decay profile of IR820 and IR820-ZnO nanohybrid monitored at 842 nm upon excitation 633 nm in DMSO. The excited state lifetime of IR820 quenches in nanohybrid as compared to that of free IR820 dye. The decay curve of IR820 is fitted with bi-exponential functions with the lifetime of 148 ps and 533.65 ps respectively. Being a NIR dye, IR820 is able to generate heat when it is exposed to NIR light.49 Besides this, it generates singlet oxygen through energy transfer to triplet oxygen from its T1 state.20, 21 Because of this phenomenon, the average lifetime of the singlet excited state of IR820 is quite faster. The average lifetime is 456.52 ps. In case of IR820ZnO nanohybrid, the emission decay curve is deviated from bi-exponential to tri-exponential and exhibit one ultrafast decay component of 39 ps (have a considerable contribution of 67.3%), which we attribute the electron transfer from LUMO (lowest unoccupied molecular orbital) of IR820 to the conduction band of ZnO.50 The electron injection process from dye to semiconductor is due to the interaction between lower-density of sp orbital of Zn+2 in ZnO with LUMO of IR820.51 The details of spectroscopic parameter and fitting parameters of fluorescence decays are provided in Table 1. This excited state photoelectron transfer from 11 ACS Paragon Plus Environment


IR820 to ZnO also confirms the attachment of IR820 dye on ZnO surface. In principle, during the electron transfer process in the nanohybrid, electrons can flow towards the semiconductor either from the singlet and triplet excited states of IR820.52, 53 In our case, the finding of a very fast 39 ps time component strongly suggests that electron transfer occurs from the S1 state.48 To further confirm that this short living fluorescence component is connected to the occurrence of photoinduced electron transfer, we have repeated the measurement of time dependent fluorescence in the absence of oxygen, to restrict the possibility of other non−radiative decay processes.54, 55 We found no significant changes in time scale in inert condition (data not shown). This observation firmly indicates that the fast time component present in IR820-ZnO is predominantly related to the electron transfer process from LUMO of IR-820 to conduction band of ZnO. This in turn decreases the rate of ISC process and prevents the formation of triplet excited state in the photosensitizer dye,56 which reduces the triplet quantum yield.57 Thus, the process of energy transfer from T1 state of IR820 to triplet state of oxygen58 (the overall dynamics is represented in the scheme 1) is reduced by electron transfer process. Hence, the singlet oxygen generation ability of IR820 on ZnO surface is reduced and the photoinduced excited state electron transfer process enhances overall generation of ROS (including superoxide) by the nanohybrid. In order to investigate the singlet oxygen mediated photodegradation, a scavenger of singlet oxygen59 (NaN3) was added with IR820 and IR820-ZnO (in this case both IR820 and IR820-ZnO nanohybrid was in the mixture of DMSO and water and monitoring the absorption peak of IR820 at 820 nm) under red light irradiation for 30 min. In presence of NaN3, the photodegradation of IR820 decreases from 42% to 27%. This is comparable with the IR820 degradation in the IR820-ZnO nanohybrid (the photodegradation of IR820-ZnO nanohybrid in DMSO and water mixture after 30 min is 24%). Whether, the degradation of the nanohybrid does not change significantly upon NaN3 addition (Fig. 5b). The details of degradation 12 ACS Paragon Plus Environment

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percentage are reported in table 2. We have further performed the singlet oxygen sensor green reagent (SOSGR) assay in presence of NaN3 under red light illumination for 20 minutes (Fig. 5c). In case of IR820, the fluorescence intensity at 530 nm reduces significantly in presence of NaN3. For IR820-ZnO nanohybrid, the decrease in intensity at 530 nm is lower than IR820. These results imply that, the greater generation of red-light-induced singlet oxygen species by IR820 reduces its photostability as compared to that of IR820-ZnO. Basically, the singlet oxygen attacks the polymethine chain of IR820 and produces fragmentation of the IR820 moiety.60 In case of IR820-ZnO nanohybrid, the electron capture process (from LUMO of IR820 to CB of ZnO) quenches the electron recombination process from triplet state to ground state (S0) in IR820. Thus, it prevents the singlet generation as well as photobleaching of IR820. Also, due to excited state electron transfer from IR820 to ZnO, the electron deficiency in IR820 protects the dye from the singlet oxygen. Finally, we can conclude that we are able to enhance the photostability and activity of a NIR cyanine dye upon attachment with ZnO nanoparticles through a very easy synthesis pathway. We believe that the enhancement of photostability of IR820 in the nanohybrid and the enhancement of overall ROS generation ability by nanohybrid will lead to the development for potential many-fold applications. In view of application in targeted therapy, we have checked the dissolution and time dependent precipitation of IR820-ZnO nanohybrids in water at different pH values. For precipitation experiment, the decrease of stability of IR820-ZnO was monitored at 370 nm (the absorption peak for ZnO in nanohybrid). At pH 5, 40% IR820-ZnO nanohybrids are precipitated after 1 hr, whereas, 17% precipitation occurs at pH7 (Fig. 6a). Thus, the dispersion of IR820-ZnO nanohybrid is less stable in acidic solution as compared to that in neutral solution. Thus, the nanohybrids are expected to be deposited more in cancer cell (cancer cells are acidic in nature) as compared to that of others cell. The dissolution of IR820-ZnO nanohybrid was monitored at 370 nm in neutral (pH 7) and acidic (pH 5) aqueous solutions. At pH 5, 42% IR820-ZnO 13 ACS Paragon Plus Environment


nanohybrids are dissolved after 24 hr, whereas, 10% dissolution occurs at pH 7 (Fig. 6b). These results show that the IR820−ZnO nanohybrid has the ability to deliver the IR820 dye selectively in cancer cells. 4. Conclusion In summary, we have explored the advantages and improved effectiveness of IR820-ZnO nanohybrid over free IR820 dye which could be translated to its potential applications in various fields. The attachment of IR820 on ZnO surface was characterised using optical spectroscopic tools. Nanohybrid provides greater photostability with a large enhancement of photoinduced ROS. Picosecond resolved fluorescence study explains the excited state electron transfer dynamics in the nanohybrid. The photoinduced excited state charge migration from IR820 to ZnO in the nanohybrid is responsible for the generation of overall ROS over free IR 820 dye. This photo-induced electron transfer process is also responsible to generate singlet oxygen deficiency in nanohybrid which subsequently improves the photostability of the nanohybrid. Our findings reveal that, for a well-known NIR cyanine dye IR820, the photostability and the ability of overall ROS generation enhances upon attachment on ZnO surface. The key advantages of IR820 attachment on ZnO surface are enhancement of photostability, restriction in thermal degradation, efficiency in ROS generation and targeted delivery towards cancer cells. The developed strategy could be applied for other NIR cyanine dyes that will improve their potential applications in light-harvesting fields.

Acknowledgements A.B. thanks the CSIR for providing fellowship. D.B. thanks the Department of Science and Technology (DST, India) for INSPIRE fellowship. We thank DST-SERB EMR/2016/004698 and DBT-BT/PR11534/NNT/28/766/2014 for financial support.


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Figure 1: (a) HRTEM image of ZnO nanoparticles. (b) Structure of IR820. (c) Scheme of formation of IR820-ZnO nanohybrid. (d) Powder XRD pattern of ZnO and IR820-ZnO nanohybrid.



Figure 2: (a) UV-VIS absorption spectra of IR820 and IR820-ZnO nanohybrid in methanol. (b) The absorption spectra of IR820 in H2O at different concentration. (c) The absorption spectra of IR820-ZnO in H2O at different concentration. (d) Room temperature emission spectra of IR820 and IR820-ZnO in methanol. Inset shows photographs of ZnO and IR820-ZnO.


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Figure 3: (a) TGA profile of IR820, IR820-ZnO and ZnO. Inset shows the enlarge graph of IR820ZnO and ZnO degradation. (b) Time dependent photodegradation of IR820-ZnO and IR820.



Figure 4: (a) DCFH oxidation (monitored at 522 nm) with time in presence of IR820-ZnO, IR820, ZnO and DCFH only under dark (10 min) and red light (30 min). Inset shows the spectra of red LED. (b) The kinetics of SOSGR (monitored at 530 nm) oxidation in presence of IR820-ZnO, IR820 and SOSGR only under dark (9 min) and red light (18 min). (c) Chemiluminescence of luminol after red light irradiation for 15 minutes in the presence of IR820-ZnO, IR820 and control luminol only.


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Figure 5: (a) Picosecond resolved fluorescence transients of IR820 and IR820-ZnO in DMSO. Inset shows steady state emission spectra of IR820 and IR820-ZnO in DMSO. (b) Time dependent photodegradation of IR820-ZnO and IR820 in presence of NaN3. (c) SOSGR oxidation kinetics in presence of IR820-ZnO and IR820 in addition of NaN3.



Figure 6: (a) Stability of nanohybrid from absorbance at 370 nm of IR820-ZnO dispersed in water at pH 5 and pH 7. (b) The dissolution of IR820-ZnO nanohybrid in water at pH 5 and pH 7.


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Scheme1: Schematic representation of electron transfer and ROS generation dynamics.



Graphical Abstract


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Table 1: Lifetime of picosecond time-resolved fluorescence transients of IR820 and IR820ZnO nanohybrid, detected at 842 nm PL maxima upon excitation at 633 nm wavelength. System

1 (ps)

τ2 (ps)

IR820

148 (20%)

533.65 (80%)

IR820-ZnO

39 (67.27%)

530 (22.73%)

τ3 (ps)

τavg (ps) 456.52

1264 (10%)

273.10

Table 2: Percentage of photodegradation of IR820 and IR820-ZnO in water-DMSO mixture in presence and absence of NaN3. System

Photodegradation

IR820

41.583%

IR820-ZnO

23.649%

IR820, NaN3

26.587%

IR820-ZnO,NaN3

21.955%


Improvement of Photostability and NIR Activity of Cyanine Dye

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