Surface chemistry modulated introduction of multifunctionality within Co₃O₄ nanocubes

Abstract

Co₃O₄ as a multifunctional nanomaterial, possessing simultaneously, unique optical, magnetic, and catalytic properties is sparse in the existing literature. We have activated intrinsic multicolor fluorescence covering the entire visible region by the functionalization of Co₃O₄ nanocubes (NCs) with Na-tartrate. The functionalized Co₃O₄ NCs show excellent catalytic efficiency in the degradation of both biologically and environmentally harmful dyes, which opens up a way for the possible application of the NCs toward therapeutic and wastewater treatment. Systematic investigations using UV-visible absorption, steady state, and time-resolved photoluminescence studies reveal the mechanistic origin behind the generation of multicolor fluorescence from the ligand-functionalized Co₃O₄ NCs. Moreover, from the magnetic study we have found that the room temperature antiferromagnetic nature of Co₃O₄ NCs turns to ferromagnetic after ligand functionalization due to the surface modification of the NCs. We believe that the developed multifunctional Co₃O₄ NCs could open up a new horizon toward their diverse applications in the present era of multitasking.

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Introduction

The aggressive scaling down of transition metal oxides to the nanometer size scale is the cornerstone for creating integrated systems with imposed multifunctionality. Although still in its infancy, the field of multifunctional nanoparticles has shown great promise to solve a number of major global issues.

The vast medical potential of nanoscale magnetic materials has revolutionized the diagnosis of ailing tissues by magnetic resonance, ultrasound,¹ and optical imaging, along with targeted drug delivery,² separation and purification of cells,³ tissue repairing,⁴ hyperthermia for cancer treatment,⁵ etc. Fluorescent magnetic nanoparticles are special candidates for many applications as they can be concurrently utilized in diagnostic and therapeutic applications. To date, fluorescent magnetic nanoparticles have been prepared either by molecular functionalization with dyes or by attachment with quantum dots (QDs). In spite of their remarkable efficiency in the field of biomedical applications, due to some serious drawbacks like photo-bleaching and the instability of organic dyes and inherent toxicity of QDs, nanoprobes with intrinsic fluorescence are highly desirable, but unfortunately, are sparsely found in the existing literature.

Advanced oxidation processes (AOPs), such as sonolysis, radiolysis, and photocatalysis, have assisted in environmental remediation by mineralizing organic compounds in aqueous media, namely by producing hydroxyl radicals as the primary oxidant.^6,7 Nanocatalysis is a versatile, low-cost, and environmentally benign technology for the treatment of a host of pollutants. The ability of AOPs to destroy low levels of persistent organic pollutants, as well as microorganisms in water, has been widely demonstrated and, progressively, the technology is now being commercialized in many areas of the world, including in developing nations. Nowadays, research has been focused in the development of nontoxic, cheaper, smarter catalysts with enhanced performance and reusability.^8–11

As an important transition metal oxide, Co₃O₄ has attracted immense attention owing to its applications in many fields, such as, catalysis,¹² toxic gas sensing,^12,13 in lithium ion batteries,¹⁴ and in supercapacitors.^15–18 The development of nanostructured Co₃O₄ with desirable size and morphologies, like cubes, rods, wires, tubes, dendrites, and sheets, have been widely explored for their superior performance in the traditional arena and also to give other unique properties.^19,20 Recently, Co₃O₄ was found to be an active agent for the photocatalytic degradation of organic pollution under light irradiation, which is a novel finding that paves the way for the exploitation and application of Co₃O₄ in such oxidative ventures.²¹ Nowadays, Co₃O₄-based nano-materials are also being promoted for biomedical applications by protein-assisted synthesis or surface modification or by designing suitable nanoarchitechtures.^22,23 However, to the best of our knowledge, the development of intrinsic fluorescence upon utilizing the surface chemistry through facile functionalization and incorporation of multifuntionality within a single Co₃O₄ nanostructure with intrinsic fluorescence, ferromagnetism, and catalytic properties, simultaneously, has not yet been reported.

Herein, we have synthesized Co₃O₄ NCs by a solvothermal route following a previous report by Zhang et al., with some modification,²⁴ where solutions of cobalt acetate in ethanol were treated with aqueous ammonium hydroxide and heated at 150 °C in an teflon-lined stainless steel autoclave for 3 h. Detailed characterization through XRD (X-ray diffraction) and TEM (Transmission electron microscopy) confirmed the formation of uniform Co₃O₄ NCs of small size (∼8 nm) with a narrow size distribution. We generated intrinsic multicolor fluorescence covering the whole visible region, ranging from blue, cyan, and green to red by functionalization of Co₃O₄ NCs with Na-tartrate. Systematic investigations through UV-visible absorption, steady state and time-resolved photoluminescence studies revealed that the ligand-to-metal charge transfer (LMCT) transition from tartrate ligand to the lowest unoccupied energy level of Co^2+/3+ of the NCs and d–d transitions centered over Co^2+/3+ ions in the NCs play the important role in the generation of multiple fluorescence from the ligand-functionalized Co₃O₄ NCs. The FTIR (Fourier transform infrared spectroscopy) study inferred the attachment of the ligand to the Co₃O₄ NCs' surface. Interestingly, we found that the magnetic behavior of the NCs can be tuned from room temperature antiferromagnetic to ferromagnetic, by surface modification of NCs with the ligands. Then, we tested the functionalized Co₃O₄ NCs in catalysis. The functionalized Co₃O₄ NCs showed very important catalytic efficiency (in the absence of any photoactivation) toward the degradation of bilirubin (BR, the pigment responsible for yellow coloration of the skin in jaundice), along with photocatalytic activity in the degradation of methylene blue (MB, a model water pollutant), which opens up possible applications toward therapeutic uses, as well as in wastewater treatment. So, by combining the manifold beneficial activities within a single entity at the nanoscale, we have developed multifunctional Co₃O₄ NCs with great potential toward diverse applications, ranging from bioimaging, drug delivery, and therapeutics, to wastewater treatment.

Results and discussion

A TEM study was carried out to evaluate the morphology and monodispersity of the solvothermally synthesized Co₃O₄ NCs. As shown in Fig. 1a, as-prepared Co₃O₄ NCs have a nearly homogeneous size distribution (6–11 nm), with an average size of 8.65 ± 0.22 nm (Fig. 1b). The corresponding SAED (selected area electron diffraction) pattern in Fig. 1c and the HRTEM (high resolution transmission electron microscopy) image in Fig. 1d confirm the crystallinity of the NCs. The calculated interplanar distance between the lattice fringes is about 0.243 nm, which corresponds to the distance between the (311) planes of Co₃O₄ crystal lattice. The EDX (energy dispersive X-ray) spectroscopic analysis of Co₃O₄ NCs (Fig. 1e) confirms the elemental composition of only cobalt, and oxygen. Fig. 1f shows the XRD pattern of the as-prepared Co₃O₄ NCs, where all the diffraction peaks in the figure perfectly match with the cubic spinel structure (JCPDS 42-1467) of Co₃O₄ NCs from the literature.²⁴

Fig. 1 (a) TEM image of the as-prepared bare Co₃O₄ NCs. (b) Size distribution of the as-prepared Co₃O₄ NCs. (c) SAED pattern of the as-prepared bare Co₃O₄ NCs (d) corresponding HRTEM image indicating the high crystallinity and showing the lattice fringes. (e) EDX spectrum of the NCs indicating the presence of only Co and O. (f) XRD pattern of as-prepared Co₃O₄ NCs. All the diffraction peaks in the figure are perfectly indexed in the literature to the cubic spinel structure of Co₃O₄ crystal lattice.

In order to solubilize Co₃O₄ NCs, we functionalized the NCs with tartrate ligands. Fig. 2a shows the TEM image of functionalized Co₃O₄ NCs with an average size of ∼7.4 nm (Fig. 2b). From the SAED pattern (lower inset of Fig. 2a) and the HRTEM image (upper inset of Fig. 2a) of functionalized Co₃O₄ NCs, the highly crystalline nature of the NCs is clearly evident. The calculated interplanar distance between the fringes was found to be 0.243 nm, corresponding to the (311) plane of the crystal lattice.

Fig. 2 (a) TEM image of highly dispersible surface-modified Co₃O₄ NCs, in aqueous medium. In the upper inset, lattice fringes in the corresponding HRTEM image indicate the highly crystalline nature of the NCs even after functionalization. In the lower inset, SAED pattern of the functionalized Co₃O₄ NCs. (b) Size distribution of Co₃O₄ NCs after solubilization in water. (c) UV-visible absorption spectrum of the functionalized Co₃O₄ NCs before and after heat and pH treatment along with only Na-tartrate.

As shown in Fig. 2c, the functionalized Co₃O₄ NCs (at pH ∼7) exhibit a distinct absorption pattern in the UV-visible region, which indicates a significant variation in the surface electronic structure of the NCs upon functionalization with the tartrate ligand. The observed peak at around 280 nm and 320 nm has a relatively higher optical density compared to another peak at 370 nm. Interestingly, upon exciting the sample at both 280 nm and 322 nm, we observed a photoluminescence peak with the maxima at 418 nm. While exciting at 355 nm, we obtained a photoluminescence peak with a maxima at 460 nm, although with a low intensity. To increase the photoluminescence intensity, we carried out further surface modification of the solubilized NCs by heating after first increasing the pH, which gave rise to the generation of two more photoluminescence peaks with multiple fold increases in the overall intensity upon excitation at proper wavelengths. The reason for the enhancement of the photoluminescence intensity, as well as for the generation of multiple optical bands upon surface modification, could be due to the increased coordination between the ligand functional groups (carboxylate and hydroxyl moieties) and the Co^2+/3+ centers at the NC surface, as evident from Fig. S2 in the ESI.†

Fig. 3a shows the normalized steady-state photoluminescence emission spectra obtained from surface-modified Co₃O₄ NCs. Upon excitation at wavelengths of 322, 355, 410, and 520 nm, the NCs solution gave rise to intense photoluminescence peaks at 418, 460, 503, and 562 nm, respectively. Photographs of the aqueous solution of surface-modified Co₃O₄ NCs under visible and UV light are presented in the inset of Fig. 3a. The amazing fluorescence micrographs, as shown in Fig. 3c, demonstrate that the black powder of functionalized Co₃O₄ NCs under a bright field generates fluorescent colors like cyan, green, and red upon excitation at 365, 436 and 546 nm, respectively, by using proper filters. Fig. S1† shows that the fluorescence micrographs of the as-prepared bare Co₃O₄ NCs under identical conditions have no such fluorescence. Fluorescence quantum yields (QY) of the functionalized Co₃O₄ NCs were calculated by following the relative method of Williams et al.,²⁵ involving the use of well-characterized standard fluorescent compounds with known QY values. The fluorescence QYs of 12% (for the 418 nm band), 1.3% (for the 460 nm band), 5.38% (for the 503 nm band), and 0.23% (for the 562 nm band) were obtained relative to the standard fluorescent compounds such as 2-aminopurine (2AP), 4′,6-diamidino-2-phenylindole (DAPI), Hoechst 33258 and Rhodamine B (RhB), respectively. Thus, the emergence of multicolor fluorescence in Co₃O₄ NCs was induced by tartrate ligand functionalization, the mystery of which can be solved by ligand field theory. The observed absorption peaks at around 285 nm and 320 nm, for functionalized Co₃O₄ NCs, which give rise to photoluminescence at 418 nm can be attributed to the LMCT involving HOMO (highest occupied molecular orbital) of the tartrate ligand and LUMO (lowest unoccupied molecular orbital) centered over the metal ion Co^2+/3+ on the NCs' surface.^26,27 Additionally, three photoluminescence peaks with maxima at 460, 503, and 562 nm, against excitation at 355, 410, and 520 nm, can arise due to the possible d–d transitions involving Co^2+/3+ ions in the functionalized Co₃O₄ NCs' surface (the presence of both Co²⁺ and Co³⁺ before and after functionalization of Co₃O₄ NCs is confirmed by the XPS, i.e., X-ray photoelectron spectroscopy, study as shown in Fig. S3 in the ESI†). We can explain the generation of the three excitation bands (375, 410, and 510 nm as shown in the excitation spectra in Fig. 3b), in terms of spectroscopic term symbols, due to transitions of ³T_1g(F) → ³A_2g(F), ³T_1g(F) → ³T_1g(P), and ³T_1g(F) → ³T_2g(F), respectively, where these energy levels are obtained from the Tanabe Sugano diagram of Co²⁺.²⁶ Additional contributions to the emission peaks with maxima at 460 nm and 503 nm may also come from the d–d transitions (¹A_1g → ¹T_2g and ¹A_1g → ¹T_1g, respectively) involving the energy levels of Co³⁺, as obtained from Tanabe Sugano diagram.²⁶ Among the d–d transitions, except for the peak at around 370 nm, other excitation bands at around 410 nm and 520 nm were not observed in the absorption spectrum (Fig. 2c), presumably due to masking by the more intense LMCT bands.

Fig. 3 (a) Normalized steady-state fluorescence emission spectra collected from the surface-modified Co₃O₄ NCs with four different excitation wavelengths of 322, 355, 410, and 520 nm. Image I and II in the inset show the digital image of the aqueous surface-modified Co₃O₄ NCs solution under visible and UV light, respectively. (b) Fluorescence excitation spectra of surface-modified Co₃O₄ NCs at different emission maximum of 415, 465, 505, and 562 (inset) nm. (c) Fluorescence micrographs of surface-modified Co₃O₄ NCs powder under bright field, UV (365 nm), blue (436 nm) and green (546 nm) light irradiation. The scale bars in all the images are 500 μm.

To obtain further reliable evidence for our proposed mechanism for the appearance of multiple photoluminescence, we performed picosecond-resolved fluorescence decay transient measurements of the functionalized Co₃O₄ NCs employing the TCSPC (time-correlated single-photon counting) technique. Fig. 4a represents the time-resolved fluorescence decay transients of functionalized Co₃O₄ NCs at two different fluorescence maxima of 410 nm and 460 nm using two different pulsed diode laser excitation sources of 294 nm and 377 nm wavelengths, respectively. As evident from the figure, a significantly larger average excited-state lifetime (τ_av) of the functionalized Co₃O₄ NCs is observed for the 410 nm fluorescence band (2.81 ns) compared to that for 460 nm fluorescence band (0.79 ns), strongly suggesting a mechanistic difference in the origin of these two fluorescence peaks. The lifetime components and their corresponding weight percentages are given in Table 1. With this photoluminescence lifetime study, proposition with regard to the origin of this multiple fluorescence made from the steady state experiments are verified, and we can thus rationally conclude that the LMCT excited state is responsible for the 410 nm photoluminescence, while the 460 nm photoluminescence is attributed to d–d transitions. Moreover, we can safely state that the other two lower energy photoluminescence at 503 nm and 562 nm also correspond to other plausible d–d transitions.

Fig. 4 (a) Picosecond-resolved fluorescence transients of functionalized Co₃O₄ NCs studied at emission wavelengths of 410 nm and 460 nm upon excitation with laser sources of 294 nm and 377 nm wavelengths, respectively. (b) FTIR spectra of as-prepared Co₃O₄ NCs and functionalized Co₃O₄ NCs along with only Na-tartrate. (c) Plot of magnetization versus applied magnetic field (M–H) for functionalized Co₃O₄ NCs at 300 K. Inset shows an M–H plot for the as-prepared Co₃O₄ NCs at 300 K.

Table 1 Lifetime values of picosecond time-resolved photoluminescence transients of functionalized Co₃O₄ NCs, detected at the photoluminescence maxima of 410 nm and 460 nm upon excitation at 294 nm and 377 nm wavelengths, respectively. The relative weight percentages of the corresponding time components are mentioned in parentheses

System	Excitation wavelength, λex (nm)	Fluorescence peak, λem (nm)	τ1 (ns)	τ2 (ns)	τ3 (ns)	τav (ns)
Functionalized Co3O4 NCs	294	410	3.27 (29.1)	8.55 (18.4)	0.55 (52.5)	2.81
	377	460	1.73 (19.04)	6.9 (3.76)	0.26 (77.2)	0.79

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In order to affirm the attachment of tartrate ligands to the NCs' surface, a comparative FTIR spectroscopic study was carried out on bare and tartrate-functionalized Co₃O₄ NCs, along with Na-tartrate alone (Fig. 4b). Co₃O₄ NCs show two strong bands at 575 cm⁻¹ (stretching vibration of (Co³⁺)–O–(Co³⁺)₃) and 669 cm⁻¹ (stretching vibration of (Co²⁺)–(Co³⁺)–O₃),²⁸ which disappear after functionalization. In the case of tartrate, the two sharp peaks arising at 1066 cm⁻¹ and 1112 cm⁻¹ are due to the C–OH stretching modes,²⁹ and peaks at 1412 cm⁻¹ and 1622 cm⁻¹ are associated with the symmetric and asymmetric stretching modes of the carboxylate groups (COO⁻) of tartrate, respectively.³⁰ Upon interaction with the nanoparticle surface, i.e., in the case of functionalized Co₃O₄ NCs, all these different bands of tartrate are perturbed notably, accompanied with the band at 3400 cm⁻¹, which corresponds to the stretching vibrational modes of hydroxyl group (O–H),²⁹ clearly proving the involvement of both –COO⁻ and –OH groups in the functionalization process.

Fig. 4c shows the magnetic study of both the as-prepared and functionalized Co₃O₄ NCs at room temperature. The inset shows the plot of magnetization versus the applied magnetic field for the as-prepared Co₃O₄ NCs, which is approximately linear with a fine hysteresis loop with a coercivity of 532 Oe and a remanent magnetization of 0.04 emu g⁻¹; however, the maximum field applied (15 kOe) cannot saturate the magnetization, and the magnetization goes up to 0.47 emu g⁻¹. So, it is evident that the as-prepared Co₃O₄ NCs show antiferromagnetic behavior with very weak ferromagnetic nature. In spite of the antiferromagnetic behavior arising due to super exchange among each Co²⁺ ion in the tetrahedral-site and its four neighboring Co²⁺ ions of opposite spins in case of bulk Co₃O₄,³¹ the very weak ferromagnetic behavior of the as-prepared NCs can be attributed to the finite size effects leading to uncompensated surface spins.³² Moreover, the ferromagnetic nature becomes more prominent after functionalization with Na-tartrate, which can be ascribed to the ligand-mediated strong pinning of the uncompensated spins of the surface Co²⁺ ions. However, the coercivity decreases significantly to 130 Oe, along with the decrease in the saturation magnetization to 0.03 emu g⁻¹. This phenomenon can be explained by ligand field theory. The tartrate ligand containing both the σ-donor (–OH) and π-donor (–COO⁻) functional groups,³³ favors quenching of the magnetic moments of Co²⁺ ions on the surface of functionalized Co₃O₄ NCs, leading to a decrease in the saturation magnetization.³⁴ Again, quenching of the magnetic moments reduces its spin–orbit coupling, resulting in a diminution of the magnetocrystalline anisotropy, which causes a reduction in the coercivity³⁵ in the case of functionalized Co₃O₄ NCs as compared to the as-prepared NCs.

Considering recent significant progress in nanocatalysis through rational engineering and the manipulation of various materials at the nanoscale to accelerate the rate of different beneficial reactions with direct biological and environmental significance, we aimed to utilize the functionalized Co₃O₄ NCs in the degradation of the biologically relevant organic dye, bilirubin (BR), the pigment responsible for the yellow coloration of the skin in jaundice. Fig. 5a shows the degradation of BR over time, in the presence of functionalized Co₃O₄ NCs at pH ∼7, which was found to follow the first order rate equation with a kinetics rate constant (k) of 2.2 × 10⁻² min⁻¹. We also verified the reusability of the catalyst, in the degradation of BR, by adding the same amount (which was added 1st time to the reaction mixture, i.e., 7.8 μM) of BR to the reaction mixture every 100 min for up to five doses, keeping the catalyst concentration fixed (adding only once at the 1st cycle), and measured the BR decomposition rate of the different cycles by monitoring the decrease of BR absorbance at 440 nm using UV-visible spectroscopy. As shown in Fig. 5b, the plot of the relative concentration of BR versus time, up to five consecutive cycles, confirms the reusability of the functionalized Co₃O₄ NCs catalyst, with an almost consistent degradation rate. Notably, as shown in Fig. 5a, an isosbestic point was obtained at ∼370 nm due to the increase in the absorbance at 319 nm with a simultaneous decrease in the absorbance at 440 nm over time. According to the reported literature on BR, the peak at 319 nm arises due to the absorption of methylvinylmaleimide (MVM), a photo-oxidation product of BR in aqueous medium.³⁶ When we carried out the reaction in the presence of a radical quencher, EtOH, (as shown in Fig. 5c) the rate of the reaction decreased significantly, with a rate constant (k) of 7.86 × 10⁻³ min⁻¹, which indicates that the reaction follows a radical pathway. Performing the reaction in the presence of the oxidizing agent H₂O₂, (as shown in Fig. 5c), we also observed a decrease in the rate of reaction (k = 1.93 × 10⁻² min⁻¹), which can be explained by combining the redox and radical pathway. The production of the oxidized product, MVM, reveals that a reduction procedure, i.e., conversion of Co³⁺ to Co²⁺ must have happened during the reaction. However, in the presence of the strong oxidizing agent H₂O₂, the reduction process (acceptance of a single electron from BR by Co³⁺ for the conversion to Co²⁺) via a radical pathway was overcompensated by the oxidation of Co²⁺ to Co³⁺, leading to an overall slowing down of the reaction. This reveals that functionalized Co₃O₄ NCs-assisted BR degradation takes place via a radical pathway involving the reduction of Co³⁺ to Co²⁺.

Fig. 5 (a) UV-visible spectral changes of an aqueous solution of bilirubin (BR) in the presence of functionalized Co₃O₄ NCs (at pH ∼7) over time. (b) Plot of the relative concentration of BR monitored at 440 nm versus time for consecutive five cycles, showing the reusability of the functionalized Co₃O₄ NCs in BR degradation. (c) The comparative rate of degradation of BR (monitored at 440 nm) alone and in the presence of functionalized Co₃O₄ NCs, functionalized Co₃O₄ NCs@EtOH, and functionalized Co₃O₄ NCs@H₂O₂.

Furthermore, inspired by the significant efficiency of functionalized Co₃O₄ NCs in the degradation of biologically harmful pigments, we attempted to utilize the strong excitation of the functionalized Co₃O₄ NCs in the UV region in photocatalysis for wastewater treatment. Functionalized Co₃O₄ NCs showed excellent photocatalytic property (as shown in Fig. 6a) toward the degradation of methylene blue, a model water-contaminant and commonly used dye in textile industries, upon UV light irradiation. We found that the functionalized Co₃O₄ NCs catalyzed photodegradation of MB takes place exponentially with time, following a first-order rate equation with a kinetic rate constant (k) of 2.23 × 10⁻² min⁻¹. To check the reusability of the catalyst, we added the same dose of MB (5 μM) into the reaction mixture for up to five doses, keeping the catalyst concentration fixed (i.e., without the addition of extra catalyst after the 1st cycle). The MB decomposition rates of different cycles were monitored by measuring the decrease of MB absorbance at 660 nm with UV-visible spectroscopy. Fig. 6b demonstrates the plots of the relative concentration of MB versus time, for up to five consecutive cycles, indicating the reusability of the functionalized Co₃O₄ NCs catalyst with a consistent degradation rate. Having evidence from several other photochemical reactions, we propose that the photodegradation process may follow a reactive oxygen species (ROS)-mediated radical pathway.

Fig. 6 (a) The plot of the relative concentration of MB monitored at 660 nm versus time with and without functionalized Co₃O₄ NCs and inset shows the UV-visible spectral changes of aqueous solution of methylene blue (MB) in the presence of functionalized Co₃O₄ NCs over time under UV irradiation. (b) The plot of the relative concentration of MB monitored at 660 nm versus time for consecutive five cycles showing the reusability of the functionalized Co₃O₄ NCs in MB degradation under UV light.

Conclusions

In summary, we have demonstrated an easy fabrication procedure for multifunctional Co₃O₄ NCs. By facile functionalization of as-prepared NCs with Na-tartrate ligand, we have been able to develop intrinsic multicolor fluorescence covering almost the entire visible region. Systematic investigations through UV-visible absorption, steady state, and time-resolved photoluminescence studies reveal that the LMCT transition from the tartrate ligand to the lowest unoccupied energy level of Co^2+/3+ of the NCs and d–d transitions centered over Co^2+/3+ ions in the NCs play the important role in the generation of multiple fluorescence from the ligand-functionalized Co₃O₄ NCs. A magnetic study exhibited that the antiferromagnetic nature of Co₃O₄ NCs turns to ferromagnetic after surface modification. The newly developed magnetofluorescent Co₃O₄ NCs can open up a new horizon in the field of bioimaging and drug delivery. The functionalized Co₃O₄ NCs also show good catalytic efficiency for the degradation of BR, a biologically harmful pigment, which reveals their developing therapeutic application in severe hyperbilirubinemia. Moreover, their good photocatalytic efficiency in the degradation of MB, a model water contaminant, may give rise to their potential application in wastewater treatment.

Acknowledgements

We thank DST for financial grants. Authors would like to thank Mr Mahesh Agarwal and Dr Deepak Kumar Sinha from IACS, Kolkata for providing fluorescence microscopy facility used in this study. Authors would like to thank Mr Sukanta Barman and Dr Krishnakumar S. R. Menon from SINP, Kolkata for providing XPS facility used in this study. We thank Mr Samik Roy Moulik for performing TEM measurements.

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Footnote

† Electronic supplementary information (ESI) available: Experimental section containing material used, detailed synthesis procedure and functionalization of Co₃O₄ NCs, characterization techniques, and fluorescence micrographs of bare Co₃O₄ NCs under bright field, UV, blue, and green light irradiation. See DOI: 10.1039/c4ra12901f

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